fix(downloads): stop repeating an extension the name already carries

A voice comment's display name is the generated file name, extension
included, so appending the extension again downloaded it as
<uuid>.webm.webm. Both download paths, the single asset route and the
project zip, now append only when the name does not already end in it.

tests/api/lib-video-assets.test.ts

@@ -19,6 +19,7 @@ import {
   extractVideoKeyFromProxyUrl,
   getVideoAssetAccessContext,
   sanitizeAssetDisplayName,
+  withFileExtension,
   SAFE_BUNNY_VIDEO_ID,
 } from '@/lib/video-assets';
 import { apiRequest } from '../helpers/request';
@@ -83,6 +84,28 @@ describe('sanitizeAssetDisplayName', () => {
   });
 });
 
+// A voice comment is stored under its generated file name, so its display name
+// already ends in .webm and every download was named `<uuid>.webm.webm`.
+describe('withFileExtension', () => {
+  it('leaves a name that already ends in the extension alone', () => {
+    expect(withFileExtension('33661b60-b658-47f9-bd40-71c32a0a2fdb.webm', '.webm')).toBe(
+      '33661b60-b658-47f9-bd40-71c32a0a2fdb.webm'
+    );
+  });
+
+  it('ignores the case the extension was written in', () => {
+    expect(withFileExtension('Take 2.WEBM', '.webm')).toBe('Take 2.WEBM');
+  });
+
+  it('appends when the name carries no extension', () => {
+    expect(withFileExtension('Voice Comment', '.webm')).toBe('Voice Comment.webm');
+  });
+
+  it('appends when the name ends in a different extension', () => {
+    expect(withFileExtension('clip.mp4', '.webm')).toBe('clip.mp4.webm');
+  });
+});
+
 describe('proxy URL extraction', () => {
   it('derives the image key and file name from a canonical image URL', () => {
     expect(extractImageKeyFromProxyUrl(IMAGE_URL)).toBe(

tests/unit/lib/project-download.test.ts

@@ -1221,6 +1221,30 @@ describe('buildProjectDownloadManifest assets', () => {
     expect(manifest.files[0]?.fileName).toBe('01-Intro-asset-B roll.png');
   });
 
+  // A voice comment's display name is the stored file name, extension included.
+  it('does not repeat an extension the display name already carries', () => {
+    const manifest = buildProjectDownloadManifest(
+      'Project',
+      [
+        video({
+          versions: [],
+          assets: [
+            asset({
+              provider: VideoAssetProvider.R2_AUDIO,
+              displayName: '33661b60-b658-47f9-bd40-71c32a0a2fdb.webm',
+              sourceUrl: '/api/upload/audio/33661b60-b658-47f9-bd40-71c32a0a2fdb.webm',
+            }),
+          ],
+        }),
+      ],
+      { includeAssets: true }
+    );
+
+    expect(manifest.files[0]?.fileName).toBe(
+      '01-Intro-asset-33661b60-b658-47f9-bd40-71c32a0a2fdb.webm'
+    );
+  });
+
   it('always names a bunny asset .mp4 regardless of the source url', () => {
     const manifest = buildProjectDownloadManifest(
       'Project',