diff --git a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/Value/SelectCountry/info.mdx b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/Value/SelectCountry/info.mdx
index d58c2028121..debea5b2159 100644
--- a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/Value/SelectCountry/info.mdx
+++ b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/Value/SelectCountry/info.mdx
@@ -27,16 +27,21 @@ const MyComponent = () => {
 
 ### TransformIn and TransformOut
 
-You can use the `transformIn` and `transformOut` to transform the value before it gets displayed in the field and before it gets sent to the form. The second parameter is the country object. You may have a look at the demo below to see how it works.
+You can use the `transformIn` and `transformOut` to transform the value before it gets displayed in the field and before it gets sent to the form context. The second parameter is the country object. You may have a look at the demo below to see how it works.
 
 ```tsx
-const transformOut = (value, country) => {
-  if (value) {
-    return `${country.name} (${value})`
+import type { CountryType } from '@dnb/eufemia/extensions/forms/Field/SelectCountry'
+
+// From the Field (internal value) to the data context or event parameter
+const transformOut = (internal: string, country: CountryType) => {
+  if (internal) {
+    return `${country.name} (${internal})`
   }
 }
-const transformIn = (value) => {
-  return String(value).match(/\((.*)\)/)?.[1]
+
+// To the Field (from e.g. defaultValue)
+const transformIn = (external: unknown) => {
+  return String(external).match(/\((.*)\)/)?.[1] || 'NO'
 }
 ```
 
diff --git a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/base-fields/String/Examples.tsx b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/base-fields/String/Examples.tsx
index a0859b76e88..bdeb53ddd1c 100644
--- a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/base-fields/String/Examples.tsx
+++ b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/base-fields/String/Examples.tsx
@@ -466,10 +466,16 @@ export function TransformInAndOut() {
   return (
     <ComponentBox scope={{ Tools }}>
       {() => {
+        // From the Field (internal value) to the data context or event parameter
         const transformOut = (value) => {
           return { value, foo: 'bar' }
         }
+
+        // To the Field (from e.g. defaultValue)
         const transformIn = (data) => {
+          if (typeof data === 'string') {
+            return data
+          }
           return data?.value
         }
 
diff --git a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/feature-fields/SelectCountry/Examples.tsx b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/feature-fields/SelectCountry/Examples.tsx
index 90baa8f6e12..54837b5ce45 100644
--- a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/feature-fields/SelectCountry/Examples.tsx
+++ b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/feature-fields/SelectCountry/Examples.tsx
@@ -101,11 +101,14 @@ export function TransformInAndOut() {
   return (
     <ComponentBox scope={{ Tools }}>
       {() => {
+        // From the Field (internal value) to the data context or event parameter
         const transformOut = (value, country) => {
           if (value) {
             return country
           }
         }
+
+        // To the Field (from e.g. defaultValue)
         const transformIn = (country) => {
           return country?.iso
         }
diff --git a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/feature-fields/more-fields/Upload/info.mdx b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/feature-fields/more-fields/Upload/info.mdx
index f67dc9d6114..8aa4948e6c6 100644
--- a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/feature-fields/more-fields/Upload/info.mdx
+++ b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/feature-fields/more-fields/Upload/info.mdx
@@ -92,3 +92,71 @@ async function virusCheck(newFiles) {
   return await Promise.all(promises)
 }
 ```
+
+### TransformIn and TransformOut
+
+You can use the `transformIn` and `transformOut` properties to transform the data from the internal format to the external format and vice versa.
+
+```tsx
+import { Form, Field, Tools } from '@dnb/eufemia/extensions/forms'
+import type {
+  UploadValue,
+  UploadFileNative,
+} from '@dnb/eufemia/extensions/forms/Field/Upload'
+
+// Our external format
+type DocumentMetadata = {
+  id: string
+  fileName: string
+}
+
+const defaultValue = [
+  {
+    id: '1234',
+    fileName: 'myFile.pdf',
+  },
+] satisfies DocumentMetadata[] as unknown as UploadValue
+
+const filesCache = new Map<string, File>()
+
+// To the Field (from e.g. defaultValue)
+const transformIn = (external?: DocumentMetadata[]) => {
+  return (
+    external?.map(({ id, fileName }) => {
+      const file: File =
+        filesCache.get(id) ||
+        new File([], fileName, { type: 'images/png' })
+
+      return { id, file }
+    }) || []
+  )
+}
+
+// From the Field (internal value) to the data context or event parameter
+const transformOut = (internal?: UploadValue) => {
+  return (
+    internal?.map(({ id, file }) => {
+      if (!filesCache.has(id)) {
+        filesCache.set(id, file)
+      }
+
+      return { id, fileName: file.name }
+    }) || []
+  )
+}
+
+function MyForm() {
+  return (
+    <Form.Handler>
+      <Field.Upload
+        path="/documents"
+        transformIn={transformIn}
+        transformOut={transformOut}
+        defaultValue={defaultValue}
+      />
+
+      <Tools.Log />
+    </Form.Handler>
+  )
+}
+```
diff --git a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/getting-started.mdx b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/getting-started.mdx
index b5a61763507..ea993ecf3c1 100644
--- a/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/getting-started.mdx
+++ b/packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/getting-started.mdx
@@ -284,20 +284,20 @@ You may check out an [interactive example](/uilib/extensions/forms/Form/Handler/
 
 #### Transforming data
 
-Each [field](/uilib/extensions/forms/all-fields/) and [value](/uilib/extensions/forms/Value/) component supports transformer functions. These functions allow you to transform a value before it is processed into the form data object and vice versa:
+Each [Field.\*](/uilib/extensions/forms/all-fields/) and [Value.\*](/uilib/extensions/forms/Value/) component supports transformer functions.
+
+These functions allow you to manipulate the field value to a different format than it uses internally and vice versa.
 
 ```tsx
 <Field.String
   path="/myField"
-  transformIn={transformIn}
   transformOut={transformOut}
+  transformIn={transformIn}
 />
 ```
 
-This allows you to show a value in a different format than it is stored in the form data object.
-
-- `transformIn` (in to the field or value) transforms the internal value before it is displayed.
-- `transformOut` (out of the field) transforms the internal value before it gets forwarded to the data context or returned as e.g. the `onChange` value parameter.
+- `transformOut` (out of the `Field.*` component) transforms the internal value before it gets forwarded to the data context or returned as e.g. the `onChange` value parameter.
+- `transformIn` (in to the `Field.*` or `Value.*` component) transforms the external value before it is displayed and used internally.
 
 <Examples.Transformers />
 
@@ -311,14 +311,18 @@ You can achieve this by using the `transformIn` and `transformOut` functions:
 
 ```tsx
 import { Field } from '@dnb/eufemia/extensions/forms'
+import type { CountryType } from '@dnb/eufemia/extensions/forms/Field/SelectCountry'
 
-const transformOut = (value, country) => {
-  if (value) {
+// From the Field (internal value) to the data context or event parameter
+const transformOut = (internal, country) => {
+  if (internal) {
     return country
   }
 }
-const transformIn = (country) => {
-  return country?.iso
+
+// To the Field (from e.g. defaultValue)
+const transformIn = (external: CountryType) => {
+  return external?.iso || 'NO'
 }
 
 const MyForm = () => {
diff --git a/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/SelectCountry.tsx b/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/SelectCountry.tsx
index 3386daa24c1..c3ffd5f8193 100644
--- a/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/SelectCountry.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/SelectCountry.tsx
@@ -23,6 +23,7 @@ export type CountryFilterSet =
   | 'Nordic'
   | 'Europe'
   | 'Prioritized'
+export type { CountryType }
 
 export type Props = FieldPropsWithExtraValue<
   string,
diff --git a/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/__tests__/SelectCountry.test.tsx b/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/__tests__/SelectCountry.test.tsx
index 13918582ed5..525fe1db2c6 100644
--- a/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/__tests__/SelectCountry.test.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/__tests__/SelectCountry.test.tsx
@@ -391,11 +391,11 @@ describe('Field.SelectCountry', () => {
         return `${country.name} (${value})`
       }
     })
-    const transformIn = jest.fn((value) => {
-      return String(value).match(/\((.*)\)/)?.[1]
+    const transformIn = jest.fn((external) => {
+      return String(external).match(/\((.*)\)/)?.[1] || external
     })
-    const valueTransformIn = jest.fn((value) => {
-      return String(value).match(/\((.*)\)/)?.[1]
+    const valueTransformIn = jest.fn((internal) => {
+      return String(internal).match(/\((.*)\)/)?.[1]
     })
 
     const onSubmit = jest.fn()
@@ -434,7 +434,7 @@ describe('Field.SelectCountry', () => {
     }
 
     expect(transformOut).toHaveBeenCalledTimes(1)
-    expect(transformIn).toHaveBeenCalledTimes(4)
+    expect(transformIn).toHaveBeenCalledTimes(3)
     expect(valueTransformIn).toHaveBeenCalledTimes(2)
 
     const firstItemElement = () =>
@@ -452,7 +452,7 @@ describe('Field.SelectCountry', () => {
     )
 
     expect(transformOut).toHaveBeenCalledTimes(1)
-    expect(transformIn).toHaveBeenCalledTimes(5)
+    expect(transformIn).toHaveBeenCalledTimes(4)
     expect(valueTransformIn).toHaveBeenCalledTimes(3)
 
     expect(input).toHaveValue('Norge')
@@ -468,7 +468,7 @@ describe('Field.SelectCountry', () => {
     expect(value).toHaveTextContent('Sveits')
 
     expect(transformOut).toHaveBeenCalledTimes(4)
-    expect(transformIn).toHaveBeenCalledTimes(8)
+    expect(transformIn).toHaveBeenCalledTimes(6)
     expect(valueTransformIn).toHaveBeenCalledTimes(4)
 
     fireEvent.submit(form)
@@ -479,7 +479,7 @@ describe('Field.SelectCountry', () => {
     )
 
     expect(transformOut).toHaveBeenCalledTimes(4)
-    expect(transformIn).toHaveBeenCalledTimes(9)
+    expect(transformIn).toHaveBeenCalledTimes(7)
     expect(valueTransformIn).toHaveBeenCalledTimes(5)
 
     expect(transformOut).toHaveBeenNthCalledWith(1, 'NO', NO)
@@ -487,15 +487,13 @@ describe('Field.SelectCountry', () => {
     expect(transformOut).toHaveBeenNthCalledWith(3, 'CH', CH)
     expect(transformOut).toHaveBeenNthCalledWith(4, 'CH', CH)
 
-    expect(transformIn).toHaveBeenNthCalledWith(1, undefined)
-    expect(transformIn).toHaveBeenNthCalledWith(2, undefined)
+    expect(transformIn).toHaveBeenNthCalledWith(1, 'NO')
+    expect(transformIn).toHaveBeenNthCalledWith(2, 'NO')
     expect(transformIn).toHaveBeenNthCalledWith(3, 'Norge (NO)')
     expect(transformIn).toHaveBeenNthCalledWith(4, 'Norge (NO)')
     expect(transformIn).toHaveBeenNthCalledWith(5, 'Norge (NO)')
-    expect(transformIn).toHaveBeenNthCalledWith(6, 'Norge (NO)')
+    expect(transformIn).toHaveBeenNthCalledWith(6, 'Sveits (CH)')
     expect(transformIn).toHaveBeenNthCalledWith(7, 'Sveits (CH)')
-    expect(transformIn).toHaveBeenNthCalledWith(8, 'Sveits (CH)')
-    expect(transformIn).toHaveBeenNthCalledWith(9, 'Sveits (CH)')
 
     expect(valueTransformIn).toHaveBeenNthCalledWith(1, undefined)
     expect(valueTransformIn).toHaveBeenNthCalledWith(2, 'Norge (NO)')
@@ -504,6 +502,121 @@ describe('Field.SelectCountry', () => {
     expect(valueTransformIn).toHaveBeenNthCalledWith(5, 'Sveits (CH)')
   })
 
+  it('should support "transformIn" and "transformOut" when value is given by the data context', async () => {
+    const transformOut = jest.fn((value, country) => {
+      if (value) {
+        return `${country.name} (${value})`
+      }
+    })
+    const transformIn = jest.fn((external) => {
+      return String(external).match(/\((.*)\)/)?.[1]
+    })
+    const valueTransformIn = jest.fn((internal) => {
+      return String(internal).match(/\((.*)\)/)?.[1]
+    })
+
+    const onSubmit = jest.fn()
+
+    render(
+      <Form.Handler
+        onSubmit={onSubmit}
+        defaultData={{ country: 'Norge (NO)' }}
+      >
+        <Field.SelectCountry
+          path="/country"
+          transformIn={transformIn}
+          transformOut={transformOut}
+        />
+
+        <Value.SelectCountry
+          path="/country"
+          transformIn={valueTransformIn}
+        />
+      </Form.Handler>
+    )
+
+    const NO = {
+      cdc: '47',
+      continent: 'Europe',
+      i18n: { en: 'Norway', nb: 'Norge' },
+      iso: 'NO',
+      name: 'Norge',
+      regions: ['Scandinavia', 'Nordic'],
+    }
+
+    const CH = {
+      cdc: '41',
+      continent: 'Europe',
+      i18n: { en: 'Switzerland', nb: 'Sveits' },
+      iso: 'CH',
+      name: 'Sveits',
+    }
+
+    expect(transformOut).toHaveBeenCalledTimes(0)
+    expect(transformIn).toHaveBeenCalledTimes(1)
+    expect(valueTransformIn).toHaveBeenCalledTimes(1)
+
+    const firstItemElement = () =>
+      document.querySelectorAll('li.dnb-drawer-list__option')[0]
+
+    const form = document.querySelector('form')
+    const input = document.querySelector('input')
+    const value = document.querySelector('.dnb-forms-value-block__content')
+
+    fireEvent.submit(form)
+    expect(onSubmit).toHaveBeenCalledTimes(1)
+    expect(onSubmit).toHaveBeenLastCalledWith(
+      { country: 'Norge (NO)' },
+      expect.anything()
+    )
+
+    expect(transformOut).toHaveBeenCalledTimes(0)
+    expect(transformIn).toHaveBeenCalledTimes(2)
+    expect(valueTransformIn).toHaveBeenCalledTimes(2)
+
+    expect(input).toHaveValue('Norge')
+    expect(value).toHaveTextContent('Norge')
+
+    await userEvent.type(input, '{Backspace>10}Sveits')
+    await waitFor(() => {
+      expect(firstItemElement()).toBeInTheDocument()
+    })
+    await userEvent.click(firstItemElement())
+
+    expect(input).toHaveValue('Sveits')
+    expect(value).toHaveTextContent('Sveits')
+
+    expect(transformOut).toHaveBeenCalledTimes(3)
+    expect(transformIn).toHaveBeenCalledTimes(4)
+    expect(valueTransformIn).toHaveBeenCalledTimes(3)
+
+    fireEvent.submit(form)
+    expect(onSubmit).toHaveBeenCalledTimes(2)
+    expect(onSubmit).toHaveBeenLastCalledWith(
+      { country: 'Sveits (CH)' },
+      expect.anything()
+    )
+
+    expect(transformOut).toHaveBeenCalledTimes(3)
+    expect(transformIn).toHaveBeenCalledTimes(5)
+    expect(valueTransformIn).toHaveBeenCalledTimes(4)
+
+    expect(transformOut).toHaveBeenNthCalledWith(1, 'NO', NO)
+    expect(transformOut).toHaveBeenNthCalledWith(2, 'CH', CH)
+    expect(transformOut).toHaveBeenNthCalledWith(3, 'CH', CH)
+
+    expect(transformIn).toHaveBeenNthCalledWith(1, 'Norge (NO)')
+    expect(transformIn).toHaveBeenNthCalledWith(2, 'Norge (NO)')
+    expect(transformIn).toHaveBeenNthCalledWith(3, 'Norge (NO)')
+    expect(transformIn).toHaveBeenNthCalledWith(4, 'Sveits (CH)')
+    expect(transformIn).toHaveBeenNthCalledWith(5, 'Sveits (CH)')
+
+    expect(valueTransformIn).toHaveBeenNthCalledWith(1, 'Norge (NO)')
+    expect(valueTransformIn).toHaveBeenNthCalledWith(2, 'Norge (NO)')
+    expect(valueTransformIn).toHaveBeenNthCalledWith(3, 'Sveits (CH)')
+    expect(valueTransformIn).toHaveBeenNthCalledWith(4, 'Sveits (CH)')
+  })
+
   it('should store "displayValue" in data context', async () => {
     let dataContext = null
 
diff --git a/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/stories/SelectCountry.stories.tsx b/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/stories/SelectCountry.stories.tsx
index b32f0ff6bf2..86315700e30 100644
--- a/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/stories/SelectCountry.stories.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/Field/SelectCountry/stories/SelectCountry.stories.tsx
@@ -24,13 +24,13 @@ export function SelectCountry() {
   )
 }
 
-const transformOut = (value, country: CountryType) => {
-  if (value) {
-    return `${country.name} (${value})`
+const transformOut = (internal: string, country: CountryType) => {
+  if (internal) {
+    return `${country.name} (${internal})`
   }
 }
-const transformIn = (value) => {
-  return String(value).match(/\((.*)\)/)?.[1]
+const transformIn = (external: unknown) => {
+  return String(external).match(/\((.*)\)/)?.[1] || 'NO'
 }
 
 export function Transform() {
diff --git a/packages/dnb-eufemia/src/extensions/forms/Field/String/__tests__/String.test.tsx b/packages/dnb-eufemia/src/extensions/forms/Field/String/__tests__/String.test.tsx
index 2e36e267f57..509a660a2b5 100644
--- a/packages/dnb-eufemia/src/extensions/forms/Field/String/__tests__/String.test.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/Field/String/__tests__/String.test.tsx
@@ -263,7 +263,7 @@ describe('Field.String', () => {
       const input = document.querySelector('input')
 
       expect(input).toHaveValue('XYZ')
-      expect(transformIn).toHaveBeenCalledTimes(2)
+      expect(transformIn).toHaveBeenCalledTimes(1)
       expect(transformIn).toHaveBeenLastCalledWith('xYz')
       expect(transformOut).toHaveBeenCalledTimes(0)
       expect(onChangeProvider).toHaveBeenCalledTimes(0)
@@ -272,7 +272,7 @@ describe('Field.String', () => {
       await userEvent.type(input, '{Backspace>3}aBc')
 
       expect(input).toHaveValue('ABC')
-      expect(transformIn).toHaveBeenCalledTimes(16)
+      expect(transformIn).toHaveBeenCalledTimes(9)
       expect(transformIn).toHaveBeenLastCalledWith('abc')
       expect(transformOut).toHaveBeenCalledTimes(13)
       expect(transformOut).toHaveBeenLastCalledWith('ABc', undefined)
@@ -287,7 +287,7 @@ describe('Field.String', () => {
       await userEvent.type(input, '{Backspace>3}EfG')
 
       expect(input).toHaveValue('EFG')
-      expect(transformIn).toHaveBeenCalledTimes(29)
+      expect(transformIn).toHaveBeenCalledTimes(16)
       expect(transformIn).toHaveBeenLastCalledWith('efg')
       expect(transformOut).toHaveBeenCalledTimes(25)
       expect(transformOut).toHaveBeenLastCalledWith('EFG', undefined)
@@ -305,6 +305,9 @@ describe('Field.String', () => {
         return { value, foo: 'bar' }
       })
       const transformIn = jest.fn((data) => {
+        if (typeof data === 'string') {
+          return data
+        }
         return data?.value
       })
       const valueTransformIn = jest.fn((data) => {
@@ -327,12 +330,14 @@ describe('Field.String', () => {
       )
 
       expect(transformOut).toHaveBeenCalledTimes(1)
-      expect(transformIn).toHaveBeenCalledTimes(4)
+      expect(transformIn).toHaveBeenCalledTimes(3)
       expect(valueTransformIn).toHaveBeenCalledTimes(2)
 
       const form = document.querySelector('form')
       const input = document.querySelector('input')
 
+      expect(input).toHaveValue('A')
+
       fireEvent.submit(form)
       expect(onSubmit).toHaveBeenCalledTimes(1)
       expect(onSubmit).toHaveBeenLastCalledWith(
@@ -346,7 +351,7 @@ describe('Field.String', () => {
       )
 
       expect(transformOut).toHaveBeenCalledTimes(1)
-      expect(transformIn).toHaveBeenCalledTimes(5)
+      expect(transformIn).toHaveBeenCalledTimes(4)
       expect(valueTransformIn).toHaveBeenCalledTimes(3)
 
       expect(input).toHaveValue('A')
@@ -362,7 +367,7 @@ describe('Field.String', () => {
       ).toHaveTextContent('B')
 
       expect(transformOut).toHaveBeenCalledTimes(6)
-      expect(transformIn).toHaveBeenCalledTimes(9)
+      expect(transformIn).toHaveBeenCalledTimes(6)
       expect(valueTransformIn).toHaveBeenCalledTimes(5)
 
       fireEvent.submit(form)
@@ -378,7 +383,7 @@ describe('Field.String', () => {
       )
 
       expect(transformOut).toHaveBeenCalledTimes(6)
-      expect(transformIn).toHaveBeenCalledTimes(10)
+      expect(transformIn).toHaveBeenCalledTimes(7)
       expect(valueTransformIn).toHaveBeenCalledTimes(6)
 
       expect(transformOut).toHaveBeenNthCalledWith(1, 'A', undefined)
@@ -396,8 +401,8 @@ describe('Field.String', () => {
       )
       expect(transformOut).toHaveBeenNthCalledWith(6, 'B', undefined)
 
-      expect(transformIn).toHaveBeenNthCalledWith(1, undefined)
-      expect(transformIn).toHaveBeenNthCalledWith(2, undefined)
+      expect(transformIn).toHaveBeenNthCalledWith(1, 'A')
+      expect(transformIn).toHaveBeenNthCalledWith(2, 'A')
       expect(transformIn).toHaveBeenNthCalledWith(3, {
         foo: 'bar',
         value: 'A',
@@ -407,26 +412,14 @@ describe('Field.String', () => {
         value: 'A',
       })
       expect(transformIn).toHaveBeenNthCalledWith(5, {
-        foo: 'bar',
-        value: 'A',
-      })
-      expect(transformIn).toHaveBeenNthCalledWith(6, {
-        foo: 'bar',
-        value: undefined,
-      })
-      expect(transformIn).toHaveBeenNthCalledWith(7, {
         foo: 'bar',
         value: undefined,
       })
-      expect(transformIn).toHaveBeenNthCalledWith(8, {
-        foo: 'bar',
-        value: 'B',
-      })
-      expect(transformIn).toHaveBeenNthCalledWith(9, {
+      expect(transformIn).toHaveBeenNthCalledWith(6, {
         foo: 'bar',
         value: 'B',
       })
-      expect(transformIn).toHaveBeenNthCalledWith(10, {
+      expect(transformIn).toHaveBeenNthCalledWith(7, {
         foo: 'bar',
         value: 'B',
       })
diff --git a/packages/dnb-eufemia/src/extensions/forms/Field/String/stories/String.stories.tsx b/packages/dnb-eufemia/src/extensions/forms/Field/String/stories/String.stories.tsx
index 0e132a3a0da..fa7021baeec 100644
--- a/packages/dnb-eufemia/src/extensions/forms/Field/String/stories/String.stories.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/Field/String/stories/String.stories.tsx
@@ -6,7 +6,7 @@ export default {
   title: 'Eufemia/Extensions/Forms/String',
 }
 
-export const String = () => {
+export const StringExample = () => {
   return (
     <Flex.Stack>
       <Field.String label="Label" />
@@ -35,11 +35,11 @@ export const String = () => {
 }
 
 export const Transform = () => {
-  const transformIn = (value) => {
-    return value?.toUpperCase()
+  const transformIn = (external: unknown) => {
+    return String(external)?.toUpperCase()
   }
-  const transformOut = (value) => {
-    return value?.toLowerCase()
+  const transformOut = (internal: string) => {
+    return internal?.toLowerCase()
   }
   return (
     <Form.Handler onChange={console.log}>
diff --git a/packages/dnb-eufemia/src/extensions/forms/Field/Upload/Upload.tsx b/packages/dnb-eufemia/src/extensions/forms/Field/Upload/Upload.tsx
index 32796efcf1d..b36bdcc99db 100644
--- a/packages/dnb-eufemia/src/extensions/forms/Field/Upload/Upload.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/Field/Upload/Upload.tsx
@@ -23,6 +23,7 @@ import { useTranslation as useSharedTranslation } from '../../../../shared'
 import { SpacingProps } from '../../../../shared/types'
 import { FormError } from '../../utils'
 
+export type { UploadFile, UploadFileNative }
 export type UploadValue = Array<UploadFile | UploadFileNative>
 export type Props = Omit<
   FieldProps<UploadValue, UploadValue | undefined>,
diff --git a/packages/dnb-eufemia/src/extensions/forms/Field/Upload/__tests__/Upload.test.tsx b/packages/dnb-eufemia/src/extensions/forms/Field/Upload/__tests__/Upload.test.tsx
index 1dc6d31013a..c4a14a91047 100644
--- a/packages/dnb-eufemia/src/extensions/forms/Field/Upload/__tests__/Upload.test.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/Field/Upload/__tests__/Upload.test.tsx
@@ -1,4 +1,4 @@
-import React from 'react'
+import React, { useContext } from 'react'
 import { fireEvent, render, waitFor, screen } from '@testing-library/react'
 import { DataContext, Field, Form, Wizard } from '../../..'
 import { BYTES_IN_A_MEGA_BYTE } from '../../../../../components/upload/UploadVerify'
@@ -7,7 +7,7 @@ import { createMockFile } from '../../../../../components/upload/__tests__/testH
 import nbNOForms from '../../../constants/locales/nb-NO'
 import nbNOShared from '../../../../../shared/locales/nb-NO'
 import userEvent from '@testing-library/user-event'
-import { UploadValue } from '../Upload'
+import { UploadFileNative, UploadValue } from '../Upload'
 import { wait } from '../../../../../core/jest/jestSetup'
 
 const nbForms = nbNOForms['nb-NO']
@@ -1527,4 +1527,155 @@ describe('Field.Upload', () => {
       document.querySelectorAll('.dnb-upload__file-cell').length
     ).toBe(0)
   })
+
+  describe('transformIn and transformOut', () => {
+    type DocumentMetadata = {
+      id: string
+      fileName: string
+    }
+
+    const defaultValue = [
+      {
+        id: '1234',
+        fileName: 'myFile.pdf',
+      },
+    ] satisfies DocumentMetadata[] as unknown as UploadValue
+
+    const filesCache = new Map<string, File>()
+
+    // To the Field (from e.g. defaultValue)
+    const transformIn = (external?: DocumentMetadata[]) => {
+      return (
+        external?.map(({ id, fileName }) => {
+          const file: File = filesCache.get(id) || new File([], fileName)
+
+          return { id, file } satisfies UploadFileNative
+        }) || []
+      )
+    }
+
+    // From the Field (internal value) to the data context or event parameter
+    const transformOut = (internal?: UploadValue) => {
+      return (
+        internal?.map(({ id, file }) => {
+          if (!filesCache.has(id)) {
+            filesCache.set(id, file)
+          }
+
+          return { id, fileName: file.name } satisfies DocumentMetadata
+        }) || []
+      )
+    }
+
+    let dataContext = null
+    function LogContext() {
+      dataContext = useContext(DataContext.Context).data
+      return null
+    }
+
+    it('should render files given in data context', async () => {
+      render(
+        <Form.Handler
+          defaultData={{
+            documents: defaultValue,
+          }}
+        >
+          <Field.Upload
+            path="/documents"
+            transformIn={transformIn}
+            transformOut={transformOut}
+          />
+          <LogContext />
+        </Form.Handler>
+      )
+
+      expect(
+        document.querySelectorAll('.dnb-upload__file-cell').length
+      ).toBe(1)
+      expect(dataContext).toEqual({
+        documents: [
+          {
+            id: '1234',
+            fileName: 'myFile.pdf',
+          },
+        ],
+      })
+
+      const file = createMockFile('secondFile.png', 100, 'image/png')
+      await waitFor(() => {
+        fireEvent.drop(document.querySelector('input'), {
+          dataTransfer: {
+            files: [file],
+          },
+        })
+      })
+
+      expect(
+        document.querySelectorAll('.dnb-upload__file-cell').length
+      ).toBe(2)
+      expect(dataContext).toEqual({
+        documents: [
+          {
+            id: '1234',
+            fileName: 'myFile.pdf',
+          },
+          {
+            id: expect.any(String),
+            fileName: 'secondFile.png',
+          },
+        ],
+      })
+    })
+
+    it('should render files given by defaultValue', async () => {
+      render(
+        <Form.Handler>
+          <Field.Upload
+            path="/documents"
+            transformIn={transformIn}
+            transformOut={transformOut}
+            defaultValue={defaultValue}
+          />
+          <LogContext />
+        </Form.Handler>
+      )
+
+      expect(
+        document.querySelectorAll('.dnb-upload__file-cell').length
+      ).toBe(1)
+      expect(dataContext).toEqual({
+        documents: [
+          {
+            id: '1234',
+            fileName: 'myFile.pdf',
+          },
+        ],
+      })
+
+      const file = createMockFile('secondFile.png', 100, 'image/png')
+      await waitFor(() => {
+        fireEvent.drop(document.querySelector('input'), {
+          dataTransfer: {
+            files: [file],
+          },
+        })
+      })
+
+      expect(
+        document.querySelectorAll('.dnb-upload__file-cell').length
+      ).toBe(2)
+      expect(dataContext).toEqual({
+        documents: [
+          {
+            id: '1234',
+            fileName: 'myFile.pdf',
+          },
+          {
+            id: expect.any(String),
+            fileName: 'secondFile.png',
+          },
+        ],
+      })
+    })
+  })
 })
diff --git a/packages/dnb-eufemia/src/extensions/forms/Field/Upload/stories/Upload.stories.tsx b/packages/dnb-eufemia/src/extensions/forms/Field/Upload/stories/Upload.stories.tsx
index db2837a8397..131fd1afffb 100644
--- a/packages/dnb-eufemia/src/extensions/forms/Field/Upload/stories/Upload.stories.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/Field/Upload/stories/Upload.stories.tsx
@@ -1,5 +1,6 @@
 import { Field, Form, Tools } from '../../..'
 import { Flex } from '../../../../../components'
+import { UploadFileNative } from '../../../../../components/Upload'
 import { createRequest } from '../../../Form/Handler/stories/FormHandler.stories'
 import { UploadValue } from '../Upload'
 
@@ -167,3 +168,68 @@ export const AsyncEverything = () => {
     </Form.Handler>
   )
 }
+
+interface DocumentMetadata {
+  id: string
+  fileName: string
+}
+
+const defaultValue = [
+  {
+    id: '1234',
+    fileName: 'myFile.pdf',
+  },
+] satisfies DocumentMetadata[] as unknown as UploadValue
+
+const filesCache = new Map<string, File>()
+
+// To the Field (from e.g. defaultValue)
+const transformIn = (external?: DocumentMetadata[]) => {
+  return (
+    external?.map(({ id, fileName }) => {
+      const file: File = filesCache.get(id) || new File([], fileName)
+
+      return { id, file } satisfies UploadFileNative
+    }) || []
+  )
+}
+
+// From the Field (internal value) to the data context or event parameter
+const transformOut = (internal?: UploadValue) => {
+  return (
+    internal?.map(({ id, file }) => {
+      if (!filesCache.has(id)) {
+        filesCache.set(id, file)
+      }
+
+      return { id, fileName: file.name } satisfies DocumentMetadata
+    }) || []
+  )
+}
+
+export function TransformInAndOut() {
+  return (
+    <Form.Handler
+    // defaultData={{
+    //   documents: defaultValue,
+    // }}
+    >
+      <Flex.Stack>
+        <Field.Upload
+          label="Label"
+          placeholder="This is a Field"
+          path="/documents"
+          transformIn={transformIn}
+          transformOut={transformOut}
+          defaultValue={defaultValue}
+          onFileClick={({ fileItem }) => {
+            console.log('onFileClick', fileItem)
+          }}
+        />
+
+        <Form.SubmitButton />
+        <Tools.Log />
+      </Flex.Stack>
+    </Form.Handler>
+  )
+}
diff --git a/packages/dnb-eufemia/src/extensions/forms/hooks/__tests__/useFieldProps.test.tsx b/packages/dnb-eufemia/src/extensions/forms/hooks/__tests__/useFieldProps.test.tsx
index d2661e96c2a..bd49aceeaad 100644
--- a/packages/dnb-eufemia/src/extensions/forms/hooks/__tests__/useFieldProps.test.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/hooks/__tests__/useFieldProps.test.tsx
@@ -2639,7 +2639,7 @@ describe('useFieldProps', () => {
     it('should call "transformOut" initially when "defaultValue" is given', () => {
       const transformOut = jest.fn((v) => v + 1)
       const transformIn = jest.fn((v) => v - 1)
-      const defaultValue = 1
+      const defaultValue = 2
 
       const { result } = renderHook(
         () =>
@@ -2657,6 +2657,7 @@ describe('useFieldProps', () => {
       })
       expect(result.current.value).toEqual(1)
       expect(transformOut).toHaveBeenCalledTimes(1)
+      expect(transformIn).toHaveBeenCalledTimes(3)
     })
 
     it('should call "transformIn" and "transformOut" after "fromInput" and "toInput"', () => {
diff --git a/packages/dnb-eufemia/src/extensions/forms/hooks/__tests__/useValueProps.test.tsx b/packages/dnb-eufemia/src/extensions/forms/hooks/__tests__/useValueProps.test.tsx
index b4a69b54cfc..7f941a18690 100644
--- a/packages/dnb-eufemia/src/extensions/forms/hooks/__tests__/useValueProps.test.tsx
+++ b/packages/dnb-eufemia/src/extensions/forms/hooks/__tests__/useValueProps.test.tsx
@@ -14,7 +14,7 @@ describe('useValueProps', () => {
     it('should prepare value', () => {
       const value = 1
 
-      const transformIn = (value) => value + 1
+      const transformIn = (external: unknown) => Number(external) + 1
       const { result } = renderHook(() =>
         useValueProps({ value, transformIn })
       )
@@ -40,7 +40,7 @@ describe('useValueProps', () => {
     it('should prepare value from context', () => {
       const path = '/contextValue'
 
-      const transformIn = (value) => value + 1
+      const transformIn = (external: unknown) => Number(external) + 1
       const { result } = renderHook(
         () => useValueProps({ path, transformIn }),
         {
diff --git a/packages/dnb-eufemia/src/extensions/forms/hooks/useFieldProps.ts b/packages/dnb-eufemia/src/extensions/forms/hooks/useFieldProps.ts
index 0cdfbba0333..f40748481d7 100644
--- a/packages/dnb-eufemia/src/extensions/forms/hooks/useFieldProps.ts
+++ b/packages/dnb-eufemia/src/extensions/forms/hooks/useFieldProps.ts
@@ -129,8 +129,8 @@ export default function useFieldProps<Value, EmptyValue, Props>(
     validateInitially,
     validateUnchanged,
     continuousValidation,
-    transformIn = (value: Value) => value,
-    transformOut = (value: Value) => value,
+    transformIn = (external: unknown) => external as Value,
+    transformOut = (internal: Value) => internal,
     toInput = (value: Value) => value,
     fromInput = (value: Value) => value,
     toEvent = (value: Value) => value,
@@ -248,16 +248,17 @@ export default function useFieldProps<Value, EmptyValue, Props>(
     defaultValueRef.current = defaultValue
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [])
-  const externalValue =
-    transformers.current.transformIn(
-      useExternalValue<Value>({
-        path,
-        itemPath,
-        value: valueProp,
-        transformers,
-        emptyValue,
-      })
-    ) ?? defaultValueRef.current
+  const tmpValue = useExternalValue<Value>({
+    path,
+    itemPath,
+    value: valueProp,
+    transformers,
+    emptyValue,
+  })
+  const externalValueDeps = tmpValue
+  const externalValue = transformers.current.transformIn(
+    tmpValue ?? defaultValueRef.current
+  )
 
   // Many variables are kept in refs to avoid triggering unnecessary update loops because updates using
   // useEffect depend on them (like the external `value`)
@@ -1761,7 +1762,9 @@ export default function useFieldProps<Value, EmptyValue, Props>(
       valueRef.current = externalValue
       externalValueDidChangeRef.current = true
     }
-  }, [externalValue, hasItemPath])
+
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [externalValueDeps, hasItemPath])
 
   useUpdateEffect(() => {
     // Error or removed error for this field from the surrounding data context (by path)
@@ -1770,7 +1773,7 @@ export default function useFieldProps<Value, EmptyValue, Props>(
       validateValue()
       forceUpdate()
     }
-  }, [externalValue]) // Keep "externalValue" in the dependency list, so it will be updated when it changes
+  }, [externalValueDeps]) // Keep "externalValue" in the dependency list, so it will be updated when it changes
 
   useEffect(() => {
     // Check against the local error state,
@@ -1813,13 +1816,12 @@ export default function useFieldProps<Value, EmptyValue, Props>(
 
       // First, look for existing data in the context
       const hasValue = pointer.has(data, identifier) || identifier === '/'
-      const existingValue = transformers.current.transformIn(
+      const existingValue =
         identifier === '/'
           ? data
           : hasValue
           ? pointer.get(data, identifier)
           : undefined
-      )
 
       // If no data where found in the dataContext, look for shared data
       if (
@@ -1831,9 +1833,7 @@ export default function useFieldProps<Value, EmptyValue, Props>(
         const sharedState = createSharedState(dataContext.id)
         const hasValue = pointer.has(sharedState.data, identifier)
         if (hasValue) {
-          const sharedValue = transformers.current.transformIn(
-            pointer.get(sharedState.data, identifier)
-          )
+          const sharedValue = pointer.get(sharedState.data, identifier)
           if (sharedValue) {
             valueToStore = sharedValue as Value
           }
@@ -1845,6 +1845,8 @@ export default function useFieldProps<Value, EmptyValue, Props>(
         typeof valueToStore === 'undefined'
 
       if (hasDefaultValue) {
+        // Set the default value if it's not set yet.
+        // This takes precedence over the valueToStore.
         valueToStore = defaultValueRef.current
         defaultValueRef.current = undefined
       }
@@ -1902,10 +1904,21 @@ export default function useFieldProps<Value, EmptyValue, Props>(
           }
 
           // Keep Iterate.Array in sync with the data context
-          valueRef.current = existingValue
+          valueRef.current = existingValue as Value
         }
       }
 
+      // When an Array or Object is used as the value,
+      // and the field uses transformIn, the instance may have been changed.
+      // The "valueToStore" can be undefined,
+      // we then need to ensure we don't overwrite the existing data context value with "undefined".
+      if (
+        typeof valueToStore === 'undefined' &&
+        typeof existingValue !== 'undefined'
+      ) {
+        valueToStore = existingValue
+      }
+
       if (
         !skipEqualCheck &&
         hasValue &&
@@ -1923,9 +1936,10 @@ export default function useFieldProps<Value, EmptyValue, Props>(
         return // stop here, avoid infinite loop
       }
 
+      const valueIn = transformers.current.transformIn(valueToStore)
       const transformedValue = transformers.current.transformOut(
-        valueToStore as Value,
-        transformers.current.provideAdditionalArgs(valueToStore as Value)
+        valueIn,
+        transformers.current.provideAdditionalArgs(valueIn as Value)
       )
       if (transformedValue !== valueToStore) {
         // When the value got transformed, we want to update the internal value, and avoid an infinite loop
@@ -1981,7 +1995,7 @@ export default function useFieldProps<Value, EmptyValue, Props>(
     [
       dataContext.internalDataRef,
       dataContext.props?.emptyData,
-      externalValue, // ensure to include "externalValue" in order to properly remove errors
+      externalValueDeps, // ensure to include "externalValue" in order to properly remove errors
     ]
   )
 
diff --git a/packages/dnb-eufemia/src/extensions/forms/hooks/useValueProps.ts b/packages/dnb-eufemia/src/extensions/forms/hooks/useValueProps.ts
index c921241ca31..dcfe9e1bffd 100644
--- a/packages/dnb-eufemia/src/extensions/forms/hooks/useValueProps.ts
+++ b/packages/dnb-eufemia/src/extensions/forms/hooks/useValueProps.ts
@@ -34,9 +34,9 @@ export default function useValueProps<
     defaultValue,
     inheritVisibility,
     inheritLabel,
-    transformIn = (value: Value) => value,
-    toInput = (value: Value) => value,
-    fromExternal = (value: Value) => value,
+    transformIn = (external: unknown) => external as Value,
+    toInput = (internal: Value) => internal,
+    fromExternal = (external: Value) => external,
   } = props
 
   const transformers = useRef({