cowork-dragdrop-issue (Similar to closed #59311 issue)

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

Drag-and-drop file attachment into a Cowork chat has stopped working. Files no longer attach when dragged from Windows Explorer into the chat window. Attaching via the (+) file picker button still works normally and uploads correctly.

This worked fine until approximately August 21, 2026, and stopped without any local change I'm aware of (no manual app reinstall, no intentional settings change). The app was already on the latest version at the time it broke — updating did not introduce or coincide with a version bump, so this doesn't appear to be a version-to-version regression from a Claude Desktop release; something else changed around that date (possibly a background auto-update component, a Windows update, or third-party software on my machine).

### What Should Happen?

The file attaches to the chat and is uploaded into the session's uploads folder, same as when using the (+) picker.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

1. Open a Cowork session in Claude Desktop on Windows 11.
2. Drag a file from Windows Explorer over the chat input.
3. Drop the file.

### Claude Model

Sonnet (default)

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

1.34493.1 (255293)

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

Other

### Additional Information

Possibly related to / same root cause as #59311 ("Cowork (Windows 11): drag-and-drop files are not uploaded to the session uploads folder"), which was closed as "not planned." Filing separately because that issue doesn't note a working-then-broke regression, and this one is on the current latest build (1.34493.1) rather than an older version.
This bug happens in the Claude Desktop app's Cowork chat window (a GUI drag-and-drop into the chat input), not in any terminal or shell session.

The report names no repository file, test, or entry point; it describes a Windows 11 Claude Desktop Cowork GUI regression rather than a terminal Claude Code path. Reproduce by dragging a file from Windows Explorer into the chat input, compare it with the (+) picker, and consider it done when drag-and-drop attaches and uploads the file.

索引モデルが issue の本文から書いたものです。