feat: improve attachment headers (#13709)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Safer, consistent file downloads with automatic attachment headers and
filenames.
- Smarter MIME detection for uploads (avatars, workspace blobs, Copilot
files/transcripts).
  - Sensible default buffer limit when reading uploads.

- **Bug Fixes**
- Prevents risky content from rendering inline by forcing downloads and
adding no‑sniff protection.
- More accurate content types when original metadata is missing or
incorrect.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/transcript/service.ts

@@ -12,6 +12,7 @@ import {
   NoCopilotProviderAvailable,
   OnEvent,
   OnJob,
+  sniffMime,
 } from '../../../base';
 import { Models } from '../../../models';
 import { PromptService } from '../prompt';
@@ -85,7 +86,10 @@ export class CopilotTranscriptionService {
         `${blobId}-${idx}`,
         buffer
       );
-      infos.push({ url, mimeType: blob.mimetype });
+      infos.push({
+        url,
+        mimeType: sniffMime(buffer, blob.mimetype) || blob.mimetype,
+      });
     }
 
     const model = await this.getModel(userId);