diff --git a/packages/next/next-server/server/image-optimizer.ts b/packages/next/next-server/server/image-optimizer.ts
index b5407c63d711d..1e789489ce954 100644
--- a/packages/next/next-server/server/image-optimizer.ts
+++ b/packages/next/next-server/server/image-optimizer.ts
@@ -198,7 +198,9 @@ export async function imageOptimizer(
 
       res.statusCode = upstreamRes.status
       upstreamBuffer = Buffer.from(await upstreamRes.arrayBuffer())
-      upstreamType = upstreamRes.headers.get('Content-Type')
+      upstreamType =
+        detectContentType(upstreamBuffer) ||
+        upstreamRes.headers.get('Content-Type')
       maxAge = getMaxAge(upstreamRes.headers.get('Cache-Control'))
     } else {
       try {
@@ -252,7 +254,8 @@ export async function imageOptimizer(
         res.statusCode = mockRes.statusCode
 
         upstreamBuffer = Buffer.concat(resBuffers)
-        upstreamType = mockRes.getHeader('Content-Type')
+        upstreamType =
+          detectContentType(upstreamBuffer) || mockRes.getHeader('Content-Type')
         maxAge = getMaxAge(mockRes.getHeader('Cache-Control'))
       } catch (err) {
         res.statusCode = 500
@@ -273,7 +276,6 @@ export async function imageOptimizer(
         return { finished: true }
       }
 
-      // If upstream type is not a valid image type, return 400 error.
       if (!upstreamType.startsWith('image/')) {
         res.statusCode = 400
         res.end("The requested resource isn't a valid image.")
@@ -426,6 +428,38 @@ function parseCacheControl(str: string | null): Map<string, string> {
   return map
 }
 
+/**
+ * Inspects the first few bytes of a buffer to determine if
+ * it matches the "magic number" of known file signatures.
+ * https://en.wikipedia.org/wiki/List_of_file_signatures
+ */
+function detectContentType(buffer: Buffer) {
+  if ([0xff, 0xd8, 0xff].every((b, i) => buffer[i] === b)) {
+    return JPEG
+  }
+  if (
+    [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a].every(
+      (b, i) => buffer[i] === b
+    )
+  ) {
+    return PNG
+  }
+  if ([0x47, 0x49, 0x46, 0x38].every((b, i) => buffer[i] === b)) {
+    return GIF
+  }
+  if (
+    [0x52, 0x49, 0x46, 0x46, 0, 0, 0, 0, 0x57, 0x45, 0x42, 0x50].every(
+      (b, i) => !b || buffer[i] === b
+    )
+  ) {
+    return WEBP
+  }
+  if ([0x3c, 0x3f, 0x78, 0x6d, 0x6c].every((b, i) => buffer[i] === b)) {
+    return SVG
+  }
+  return null
+}
+
 export function getMaxAge(str: string | null): number {
   const minimum = 60
   const map = parseCacheControl(str)
diff --git a/test/integration/image-optimizer/test/index.test.js b/test/integration/image-optimizer/test/index.test.js
index 2a5d8ecdaab14..e553e0a31dc19 100644
--- a/test/integration/image-optimizer/test/index.test.js
+++ b/test/integration/image-optimizer/test/index.test.js
@@ -330,6 +330,26 @@ function runTests({ w, isDev, domains }) {
       expect(res.headers.get('etag')).toBeTruthy()
       await expectWidth(res, w)
     })
+
+    it('should automatically detect image type when content-type is octet-stream', async () => {
+      const url =
+        'https://image-optimization-test.vercel.app/png-as-octet-stream'
+      const resOrig = await fetch(url)
+      expect(resOrig.status).toBe(200)
+      expect(resOrig.headers.get('Content-Type')).toBe(
+        'application/octet-stream'
+      )
+      const query = { url, w, q: 80 }
+      const opts = { headers: { accept: 'image/webp' } }
+      const res = await fetchViaHTTP(appPort, '/_next/image', query, opts)
+      expect(res.status).toBe(200)
+      expect(res.headers.get('Content-Type')).toBe('image/webp')
+      expect(res.headers.get('cache-control')).toBe(
+        'public, max-age=0, must-revalidate'
+      )
+      expect(res.headers.get('etag')).toBeTruthy()
+      await expectWidth(res, w)
+    })
   }
 
   it('should fail when url has file protocol', async () => {
@@ -697,7 +717,11 @@ describe('Image Optimizer', () => {
   })
 
   // domains for testing
-  const domains = ['localhost', 'example.com']
+  const domains = [
+    'localhost',
+    'example.com',
+    'image-optimization-test.vercel.app',
+  ]
 
   describe('dev support w/o next.config.js', () => {
     const size = 384 // defaults defined in server/config.ts