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/storage/providers/index.ts

@@ -135,4 +135,4 @@ export const StorageJSONSchema: JSONSchema = {
 };
 
 export type * from './provider';
-export { autoMetadata, toBuffer } from './utils';
+export { applyAttachHeaders, autoMetadata, sniffMime, toBuffer } from './utils';

packages/backend/server/src/base/storage/providers/utils.ts

@@ -1,6 +1,7 @@
 import { Readable } from 'node:stream';
 
 import { crc32 } from '@node-rs/crc32';
+import type { Response } from 'express';
 import { getStreamAsBuffer } from 'get-stream';
 
 import { getMime } from '../../../native';
@@ -43,4 +44,53 @@ export function autoMetadata(
   return metadata;
 }
 
+const DANGEROUS_INLINE_MIME_PREFIXES = [
+  'text/html',
+  'application/xhtml+xml',
+  'image/svg+xml',
+  'application/xml',
+  'text/xml',
+  'text/javascript',
+];
+
+export function isDangerousInlineMime(mime: string | undefined) {
+  if (!mime) return false;
+  const lower = mime.toLowerCase();
+  return DANGEROUS_INLINE_MIME_PREFIXES.some(p => lower.startsWith(p));
+}
+
+export function applyAttachHeaders(
+  res: Response,
+  options: { filename?: string; buffer?: Buffer; contentType?: string }
+) {
+  let { filename, buffer, contentType } = options;
+  res.setHeader('X-Content-Type-Options', 'nosniff');
+  if (!contentType && buffer) contentType = sniffMime(buffer);
+  if (contentType && isDangerousInlineMime(contentType)) {
+    const safeName = (filename || 'download')
+      .replace(/[\r\n]/g, '')
+      .replace(/[^\w\s.-]/g, '_');
+    res.setHeader(
+      'Content-Disposition',
+      `attachment; filename="${encodeURIComponent(safeName)}"; filename*=UTF-8''${encodeURIComponent(
+        safeName
+      )}`
+    );
+  }
+  if (!res.getHeader('Content-Type')) {
+    res.setHeader('Content-Type', contentType || 'application/octet-stream');
+  }
+}
+
+export function sniffMime(
+  buffer: Buffer,
+  declared?: string
+): string | undefined {
+  try {
+    const detected = getMime(buffer);
+    if (detected) return detected;
+  } catch {}
+  return declared;
+}
+
 export const SIGNED_URL_EXPIRED = 60 * 60; // 1 hour

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