feat: add support for audio/webm to detect-media-type (#7497)

## Background

Chrome uses WebM audio with MediaRecorder by default. Sending WebM audio
to the OpenAI transcription API was setting mediaType to audio/wav and
causing an error.

## Summary

I added support for WebM to detect-media-type. Now the WebM mediaType is
correctly set in the gpt-4o-transcribe parameters.

## Verification

I modified my local installation of the AI package version beta
5.0.0-beta.25 with the changes from this PR. I was then able to
successfully call transcribe with audio from Chrome's MediaRecorder in
my application.

Author: jakesjews

.changeset/polite-rivers-smoke.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+Add support for audio/webm to detect-media-type

packages/ai/src/util/detect-media-type.test.ts

@@ -415,6 +415,28 @@ describe('detectMediaType', () => {
     });
   });
 
+  describe('WEBM', () => {
+    it('should detect WEBM from bytes', () => {
+      const webmBytes = new Uint8Array([0x1a, 0x45, 0xdf, 0xa3]);
+      expect(
+        detectMediaType({
+          data: webmBytes,
+          signatures: audioMediaTypeSignatures,
+        }),
+      ).toBe('audio/webm');
+    });
+
+    it('should detect WEBM from base64', () => {
+      const webmBase64 = 'GkXfow=='; // Base64 string starting with WEBM signature
+      expect(
+        detectMediaType({
+          data: webmBase64,
+          signatures: audioMediaTypeSignatures,
+        }),
+      ).toBe('audio/webm');
+    });
+  });
+
   describe('error cases', () => {
     it('should return undefined for unknown image formats', () => {
       const unknownBytes = new Uint8Array([0x00, 0x01, 0x02, 0x03]);

packages/ai/src/util/detect-media-type.ts

@@ -83,6 +83,11 @@ export const audioMediaTypeSignatures = [
     bytesPrefix: [0x66, 0x74, 0x79, 0x70],
     base64Prefix: 'ZnR5cA',
   },
+  {
+    mediaType: 'audio/webm',
+    bytesPrefix: [0x1a, 0x45, 0xdf, 0xa3],
+    base64Prefix: 'GkXf',
+  },
 ] as const;
 
 const stripID3 = (data: Uint8Array | string) => {