Cowork: plugin upload dialog fails for .zip and .plugin files

Bug Report: Plugin Upload Dialog

Description

The upload dialog in Cowork (desktop app) fails to import plugins via .zip or .plugin file uploads. Previously working plugin files also fail — the dialog either silently fails or returns unclear errors.

Reproduction Steps

1. Open Cowork desktop app
2. Go to plugin management
3. Attempt to upload a .zip or .plugin file containing a valid plugin
4. Observe: upload fails silently or with unhelpful error

Expected Behavior

Plugin should be imported and available in the Customize panel and chat.

Workaround

Use GitHub marketplace sync (repo-based install) instead of file upload.

Environment

• Cowork desktop app (macOS)
• Plugin format confirmed valid (works in Claude Code CLI)