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/__tests__/copilot.e2e.ts

@@ -30,6 +30,7 @@ import {
   createTestingApp,
   createWorkspace,
   inviteUser,
+  smallestPng,
   TestingApp,
   TestUser,
 } from './utils';
@@ -453,8 +454,6 @@ test('should create message correctly', async t => {
         randomUUID(),
         textPromptName
       );
-      const smallestPng =
-        'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAIAQMAAAD+wSzIAAAABlBMVEX///+/v7+jQ3Y5AAAADklEQVQI12P4AIX8EAgALgAD/aNpbtEAAAAASUVORK5CYII';
       const pngData = await fetch(smallestPng).then(res => res.arrayBuffer());
       const messageId = await createCopilotMessage(
         app,
@@ -475,8 +474,6 @@ test('should create message correctly', async t => {
         randomUUID(),
         textPromptName
       );
-      const smallestPng =
-        'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAIAQMAAAD+wSzIAAAABlBMVEX///+/v7+jQ3Y5AAAADklEQVQI12P4AIX8EAgALgAD/aNpbtEAAAAASUVORK5CYII';
       const pngData = await fetch(smallestPng).then(res => res.arrayBuffer());
       const messageId = await createCopilotMessage(
         app,

packages/backend/server/src/__tests__/user/user.e2e.ts

@@ -6,6 +6,8 @@ import ava from 'ava';
 import {
   createTestingApp,
   getPublicUserById,
+  smallestGif,
+  smallestPng,
   TestingApp,
   updateAvatar,
 } from '../utils';
@@ -27,7 +29,9 @@ test('should be able to upload user avatar', async t => {
   const { app } = t.context;
 
   await app.signup();
-  const avatar = Buffer.from('test');
+  const avatar = await fetch(smallestPng)
+    .then(res => res.arrayBuffer())
+    .then(b => Buffer.from(b));
   const res = await updateAvatar(app, avatar);
 
   t.is(res.status, 200);
@@ -36,19 +40,23 @@ test('should be able to upload user avatar', async t => {
 
   const avatarRes = await app.GET(new URL(avatarUrl).pathname);
 
-  t.deepEqual(avatarRes.body, Buffer.from('test'));
+  t.deepEqual(avatarRes.body, avatar);
 });
 
 test('should be able to update user avatar, and invalidate old avatar url', async t => {
   const { app } = t.context;
 
   await app.signup();
-  const avatar = Buffer.from('test');
+  const avatar = await fetch(smallestPng)
+    .then(res => res.arrayBuffer())
+    .then(b => Buffer.from(b));
   let res = await updateAvatar(app, avatar);
 
   const oldAvatarUrl = res.body.data.uploadAvatar.avatarUrl;
 
-  const newAvatar = Buffer.from('new');
+  const newAvatar = await fetch(smallestGif)
+    .then(res => res.arrayBuffer())
+    .then(b => Buffer.from(b));
   res = await updateAvatar(app, newAvatar);
   const newAvatarUrl = res.body.data.uploadAvatar.avatarUrl;
 
@@ -58,14 +66,16 @@ test('should be able to update user avatar, and invalidate old avatar url', asyn
   t.is(avatarRes.status, 404);
 
   const newAvatarRes = await app.GET(new URL(newAvatarUrl).pathname);
-  t.deepEqual(newAvatarRes.body, Buffer.from('new'));
+  t.deepEqual(newAvatarRes.body, newAvatar);
 });
 
 test('should be able to get public user by id', async t => {
   const { app } = t.context;
 
   const u1 = await app.signup();
-  const avatar = Buffer.from('test');
+  const avatar = await fetch(smallestPng)
+    .then(res => res.arrayBuffer())
+    .then(b => Buffer.from(b));
   await updateAvatar(app, avatar);
   const u2 = await app.signup();
 

packages/backend/server/src/__tests__/utils/blobs.ts

@@ -3,6 +3,10 @@ import { type Blob } from '@prisma/client';
 import { TestingApp } from './testing-app';
 import { TEST_LOG_LEVEL } from './utils';
 
+export const smallestPng =
+  'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAIAQMAAAD+wSzIAAAABlBMVEX///+/v7+jQ3Y5AAAADklEQVQI12P4AIX8EAgALgAD/aNpbtEAAAAASUVORK5CYII';
+export const smallestGif = 'data:image/gif;base64,R0lGODlhAQABAAAAACw=';
+
 export async function listBlobs(
   app: TestingApp,
   workspaceId: string

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

packages/backend/server/src/core/user/controller.ts

@@ -1,7 +1,11 @@
 import { Controller, Get, Param, Res } from '@nestjs/common';
 import type { Response } from 'express';
 
-import { ActionForbidden, UserAvatarNotFound } from '../../base';
+import {
+  ActionForbidden,
+  applyAttachHeaders,
+  UserAvatarNotFound,
+} from '../../base';
 import { Public } from '../auth/guard';
 import { AvatarStorage } from '../storage';
 
@@ -30,6 +34,10 @@ export class UserAvatarController {
       res.setHeader('last-modified', metadata.lastModified.toISOString());
       res.setHeader('content-length', metadata.contentLength);
     }
+    applyAttachHeaders(res, {
+      contentType: metadata?.contentType,
+      filename: `${id}`,
+    });
 
     body.pipe(res);
   }

packages/backend/server/src/core/user/resolver.ts

@@ -17,6 +17,8 @@ import { isNil, omitBy } from 'lodash-es';
 import {
   CannotDeleteOwnAccount,
   type FileUpload,
+  readBufferWithLimit,
+  sniffMime,
   Throttle,
   UserNotFound,
 } from '../../base';
@@ -98,20 +100,20 @@ export class UserResolver {
     @Args({ name: 'avatar', type: () => GraphQLUpload })
     avatar: FileUpload
   ) {
-    if (!avatar.mimetype.startsWith('image/')) {
-      throw new Error('Invalid file type');
-    }
-
     if (!user) {
       throw new UserNotFound();
     }
 
+    const avatarBuffer = await readBufferWithLimit(avatar.createReadStream());
+    const contentType = sniffMime(avatarBuffer, avatar.mimetype);
+    if (!contentType || !contentType.startsWith('image/')) {
+      throw new Error(`Invalid file type: ${contentType || 'unknown'}`);
+    }
+
     const avatarUrl = await this.storage.put(
       `${user.id}-avatar-${Date.now()}`,
-      avatar.createReadStream(),
-      {
-        contentType: avatar.mimetype,
-      }
+      avatarBuffer,
+      { contentType }
     );
 
     if (user.avatarUrl) {

packages/backend/server/src/core/workspaces/controller.ts

@@ -2,6 +2,7 @@ import { Controller, Get, Logger, Param, Query, Res } from '@nestjs/common';
 import type { Response } from 'express';
 
 import {
+  applyAttachHeaders,
   BlobNotFound,
   CallMetric,
   CommentAttachmentNotFound,
@@ -83,6 +84,10 @@ export class WorkspacesController {
     } else {
       this.logger.warn(`Blob ${workspaceId}/${name} has no metadata`);
     }
+    applyAttachHeaders(res, {
+      contentType: metadata?.contentType,
+      filename: name,
+    });
 
     res.setHeader('cache-control', 'public, max-age=2592000, immutable');
     body.pipe(res);
@@ -215,6 +220,10 @@ export class WorkspacesController {
         `Comment attachment ${workspaceId}/${docId}/${key} has no metadata`
       );
     }
+    applyAttachHeaders(res, {
+      contentType: metadata?.contentType,
+      filename: key,
+    });
 
     res.setHeader('cache-control', 'private, max-age=2592000, immutable');
     body.pipe(res);

packages/backend/server/src/plugins/copilot/context/resolver.ts

@@ -31,6 +31,7 @@ import {
   EventBus,
   type FileUpload,
   RequestMutex,
+  sniffMime,
   Throttle,
   TooManyRequest,
   UserFriendlyError,
@@ -671,7 +672,11 @@ export class CopilotContextResolver {
       const { filename, mimetype } = content;
 
       await this.storage.put(user.id, session.workspaceId, blobId, buffer);
-      const file = await session.addFile(blobId, filename, mimetype);
+      const file = await session.addFile(
+        blobId,
+        filename,
+        sniffMime(buffer, mimetype) || mimetype
+      );
 
       await this.jobs.addFileEmbeddingQueue({
         userId: user.id,

packages/backend/server/src/plugins/copilot/controller.ts

@@ -32,6 +32,7 @@ import {
 } from 'rxjs';
 
 import {
+  applyAttachHeaders,
   BlobNotFound,
   CallMetric,
   Config,
@@ -795,6 +796,10 @@ export class CopilotController implements BeforeApplicationShutdown {
     } else {
       this.logger.warn(`Blob ${workspaceId}/${key} has no metadata`);
     }
+    applyAttachHeaders(res, {
+      contentType: metadata?.contentType,
+      filename: key,
+    });
 
     res.setHeader('cache-control', 'public, max-age=2592000, immutable');
     body.pipe(res);

packages/backend/server/src/plugins/copilot/resolver.ts

@@ -30,6 +30,7 @@ import {
   Paginated,
   PaginationInput,
   RequestMutex,
+  sniffMime,
   Throttle,
   TooManyRequest,
   UserFriendlyError,
@@ -806,7 +807,10 @@ export class CopilotResolver {
           filename,
           uploaded.buffer
         );
-        attachments.push({ attachment, mimeType: blob.mimetype });
+        attachments.push({
+          attachment,
+          mimeType: sniffMime(uploaded.buffer, blob.mimetype) || blob.mimetype,
+        });
       }
     }
 

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);

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

@@ -2,7 +2,12 @@ import { createHash } from 'node:crypto';
 
 import { Injectable, OnApplicationBootstrap } from '@nestjs/common';
 
-import { FileUpload, JobQueue, PaginationInput } from '../../../base';
+import {
+  FileUpload,
+  JobQueue,
+  PaginationInput,
+  sniffMime,
+} from '../../../base';
 import { ServerFeature, ServerService } from '../../../core';
 import { Models } from '../../../models';
 import { CopilotStorage } from '../storage';
@@ -64,7 +69,7 @@ export class CopilotWorkspaceService implements OnApplicationBootstrap {
     const file = await this.models.copilotWorkspace.addFile(workspaceId, {
       fileName,
       blobId,
-      mimeType: content.mimetype,
+      mimeType: sniffMime(buffer, content.mimetype) || content.mimetype,
       size: buffer.length,
     });
     return { blobId, file };