NFT metadata extension tag

CIP-0025/README.md

@@ -42,7 +42,7 @@ This is the registered `transaction_metadatum_label` value
 
 The structure allows for multiple token mints, also with different policies, in a single transaction.
 
-```
+```json
 {
   "721": {
     "<policy_id>": {
@@ -61,10 +61,11 @@ The structure allows for multiple token mints, also with different policies, in
           <other_properties>
         }],
 
-        <other properties>
+        <other properties> // overwritten by "ext" if defined
       }
     },
     "version": <version_id>
+    "ext": <array>
   }
 }
 ```
@@ -84,17 +85,19 @@ The structure allows for multiple token mints, also with different policies, in
 
 - The **`mediaType`** and **`files`** properties are optional.<br /> **`mediaType`** is however required inside **`files`**. The **`src`** property inside **`files`** is an URI pointing to a resource of this mime type. If the mime type is `image/*`, **`mediaType`** points to the same image, like the on in the **`image`** property, but in an higher resolution.
 
-- The **`version`** property is also optional. If not specified the version is `1`. It will become mandatory in future versions if needed.
+- The **`ext`** property is optional. It is used to extend known metadata properties.
+
+- The **`version`** property is optional. If not specified the version is `1`. It will become mandatory in future versions if needed.
 
-- This structure really just defines the basis. New properties and standards can be defined later on for varies uses cases. That's why there is an "other properties" tag.
+- This structure really just defines the basis. New properties and standards can be defined later on for varies uses cases. That's why there is an "other properties" tag. Note "other properties" are always overwritten by properties defined by "ext".
 
 - The retrieval of the metadata should be the same for all however.
 
 Optional fields allow to save space in the blockchain. Consequently the minimal structure for a single token is:
 
 #### Version 1
 
-```
+```json
 {
   "721": {
     "<policy_id>": {
@@ -109,7 +112,7 @@ Optional fields allow to save space in the blockchain. Consequently the minimal
 
 #### Version 2
 
-```
+```json
 {
   "721": {
     "<policy_id>": {
@@ -139,7 +142,7 @@ Using the latest mint transaction with the label 721 as valid metadata for a tok
 
 ## Backward Compatibility
 
-To keep NFT metadata compatible with changes coming up in the future, we use the **`version`** property.
+To keep NFT metadata compatible with changes coming up in the future, we use the **`version`** and **`ext`** properties.
 A future version will introduce [schema.org](https://schema.org).
 
 ## References

NFT-EXT-PROPERTY/README.md

@@ -0,0 +1,58 @@
+---
+CIP: To be confirmed?
+Title: 721 Metatdatum extension property
+Authors: Jack ([9000] pool, https://forum.cardano.org/u/jack7e/, @ada9000_ twitter)
+Comments-URI: https://github.com/cardano-foundation/CIPs/pull/249
+Status: Draft
+Type: Informational
+Created: 2022-09-28
+---
+
+# Abstract
+
+This standard proposes a method to define optional extensions the 721 metadatum standard. Allowing users to know what to expect in the metadata.
+
+# Motivation
+
+- Cardano NFT Metadata parsing is becoming complex due to optional metadata json tags.
+- If Cardano NFT's want to develop we need to define what's inside the metadata to avoid conflicts with custom optional tags and tags defined in new CIPs.
+
+# Specification
+
+- The `ext` tag is added within the `"721"` metadatum tag
+- The `ext` tag should be lowercase. When parsing nft metadata the developer will use a to lowercase function on all elements in the `ext` array
+
+```json
+{
+   "721":{
+      "ext": <array>
+      "<policy_id>":{
+         "<asset_name>": {
+            "name": <string>,
+            "image": <uri | array>
+         }
+      }
+   }
+}
+```
+
+# Example usage
+
+The theoretical cipX in this example defines a script tag that must be present. To implement this CIP in our 721 metadatum JSON we simply add CIPX to the `"ext"` array.
+
+```json
+{
+   "721":{
+      "ext": ["cipX"]
+      "<policy_id>":{
+         "script":"<script_policy_id>"
+         "<asset_name>": {
+            "name": <string>,
+            "image": <uri | array>
+         }
+      }
+   }
+}
+```
+
+Where cipX defines some property for the **JSON**. In this particular example cipX is a fictional CIP that requires the nft to contain a `"script"` tag with a `"<script_policy_id>"`.