fix(uploads): persist thread uploads canonically and fail fast on upload errors (#943)

* fix(uploads): persist thread uploads canonically and fail fast on upload errors

 - write uploads to thread-scoped storage first to guarantee agent visibility
 - sync files to sandbox virtual path only for non-local sandboxes
 - fix markdown conversion flow to operate on canonical saved files and sync converted files when needed
 - prevent silent attachment upload failures in frontend submit flow (show error + abort submit)
 - add regression tests for local vs non-local upload behavior
 - update upload docs with thread-first persistence and troubleshooting notes

* Update frontend/src/core/threads/hooks.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix(uploads): reject "." and ".." filenames in upload sanitization (#944)

* Initial plan

* fix(uploads): reject '.' and '..' filenames in upload sanitization

Co-authored-by: WillemJiang <219644+WillemJiang@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: WillemJiang <219644+WillemJiang@users.noreply.github.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>

backend/docs/FILE_UPLOAD.md

@@ -131,6 +131,11 @@ read_file(path="/mnt/user-data/uploads/document.md")
 - 实际存储：`backend/.deer-flow/threads/{thread_id}/user-data/uploads/document.pdf`
 - 前端访问：`/api/threads/{thread_id}/artifacts/mnt/user-data/uploads/document.pdf`（HTTP URL）
 
+上传流程采用“线程目录优先”策略：
+- 先写入 `backend/.deer-flow/threads/{thread_id}/user-data/uploads/` 作为权威存储
+- 本地沙箱（`sandbox_id=local`）直接使用线程目录内容
+- 非本地沙箱会额外同步到 `/mnt/user-data/uploads/*`，确保运行时可见
+
 ## 测试示例
 
 ### 使用 curl 测试
@@ -243,7 +248,8 @@ backend/.deer-flow/threads/
 
 1. 确认 UploadsMiddleware 已在 agent.py 中注册
 2. 检查 thread_id 是否正确
-3. 确认文件确实已上传到正确的目录
+3. 确认文件确实已上传到 `backend/.deer-flow/threads/{thread_id}/user-data/uploads/`
+4. 非本地沙箱场景下，确认上传接口没有报错（需要成功完成 sandbox 同步）
 
 ## 开发建议