fix(ai): name data-URL audio with its real extension

A data URL's filename came from its MIME subtype, so `audio/mpeg`
became `input.mpeg` and `audio/x-wav` became `input.x-wav`. OpenAI's
transcription API picks the decoder from the extension and rejected
both. Map the MIME types whose subtype isn't the extension.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/backend/drivers/util/fileInput.test.ts

@@ -237,6 +237,24 @@ describe('loadFileInput data URL', () => {
         expect(result.mimeType).toBe('image/svg+xml');
     });
 
+    // Audio upstreams pick the decoder from the extension; `input.mpeg` is
+    // rejected as corrupt.
+    it.each([
+        ['audio/mpeg', 'input.mp3'],
+        ['audio/x-wav', 'input.wav'],
+        ['audio/wave', 'input.wav'],
+        ['audio/x-m4a', 'input.m4a'],
+        ['audio/ogg', 'input.ogg'],
+    ])('names a %s data URL %s', async (mime, filename) => {
+        const { actor } = await makeUser();
+        const result = await callLoadFileInput(
+            actor,
+            `data:${mime};base64,QUJD`,
+        );
+        expect(result.filename).toBe(filename);
+        expect(result.mimeType).toBe(mime);
+    });
+
     it('defaults MIME to application/octet-stream when omitted', async () => {
         const { actor } = await makeUser();
         const result = await callLoadFileInput(actor, 'data:;base64,QUJD');

src/backend/drivers/util/fileInput.ts

@@ -321,8 +321,22 @@ function assertMax(buffer: Buffer, maxBytes?: number): void {
     }
 }
 
+/**
+ * MIME types whose subtype isn't the file extension. Upstreams like OpenAI's
+ * audio API pick the decoder from the extension and reject `input.mpeg`.
+ */
+const EXT_BY_MIME: Record<string, string> = {
+    'audio/mpeg': 'mp3',
+    'audio/x-wav': 'wav',
+    'audio/wave': 'wav',
+    'audio/x-m4a': 'm4a',
+};
+
 function filenameFromMime(mime: string): string {
-    const ext = mime.split('/')[1]?.split('+')[0] ?? 'bin';
+    const ext =
+        EXT_BY_MIME[mime.toLowerCase()] ??
+        mime.split('/')[1]?.split('+')[0] ??
+        'bin';
     return `input.${ext}`;
 }