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/base/utils/stream.ts

@@ -1,6 +1,7 @@
 import { Readable } from 'node:stream';
 
 import { BlobQuotaExceeded, StorageQuotaExceeded } from '../error';
+import { OneKB } from './unit';
 
 export type CheckExceededResult =
   | {
@@ -52,7 +53,7 @@ export async function readBuffer(
 
 export async function readBufferWithLimit(
   readable: Readable,
-  limit: number
+  limit: number = 500 * OneKB
 ): Promise<Buffer> {
   return readBuffer(readable, size =>
     size > limit