[BUG] Cowork: in-session file attach picker grays out ALL files (every type), making selection impossible

Preflight Checklist

• [x] I have searched existing issues 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?

Summary

In a Claude Desktop Cowork session, the "+" / attach-file picker behaves differently before vs. after the session goes active, and the in-session version is broken:

Before sending the first prompt (new task, composer view): clicking the attach button ("+", folder-with-plus icon) opens the macOS file picker and I can select any file normally. Works as it always has.
After the first prompt is sent and the chat is active: clicking the same attach button opens the macOS file picker, but every file is grayed out and unselectable — across all file types (PNG, PDF, .docx, everything). Nothing is selectable.

Drag-and-drop of the same files into the active chat works fine, which confirms the files and folder permissions are fine — the problem is isolated to the in-session picker's allowed-file-type filter.

Environment

App: Claude Desktop (Cowork mode)
App version: Claude 1.15962.1 (1e236d) 2026-06-26T19:44:44.000Z
OS: macOS Version 26.5.1 (25F80)
Onset: started after an app auto-update on the morning of 2026-06-27. Picker worked correctly on the prior build.

Steps to reproduce

Start a new Cowork task and assign a project.
In the composer (before sending), click the attach ("+") button → file picker opens → files are selectable. ✅
Send the first prompt; wait for the session to become active.
In the active chat, click the attach ("+") button → file picker opens → all files are grayed out / unselectable, regardless of type. ❌

Expected

The in-session attach picker should allow selecting files, identical to the pre-session composer picker.

Actual

Every file in the picker is disabled (grayed out) — no file of any type can be selected. Only drag-and-drop works as a bypass.

Likely root cause

A macOS open panel grays out files that don't match the allowedContentTypes / allowed file-type filter passed by the calling app. Because the pre-session and in-session pickers are separate code paths, and because all file types are grayed (not just a subset), the in-session picker appears to be passing an empty or invalid allowed-types list, which the OS resolves to "nothing matches" and disables every file. This looks like a regression in the build that auto-installed on 2026-06-27.

Workaround

Drag-and-drop files directly into the active chat (bypasses the picker entirely).

Notes

This may be related to the recent wave of Cowork desktop file/folder plumbing regressions after app updates (e.g. anthropics/claude-code#25797, where the folder-picker tool went missing from sessions after a desktop update). I did not find an existing report matching this exact symptom (in-session attach picker graying all files).

What Should Happen?

see above

Error Messages/Logs

Steps to Reproduce

see above

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude 1.15962.1 (1e236d) 2026-06-26T19:44:44.000Z

Platform

Other

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_