[Bug] Claude Desktop plugin upload rejects `.plugin` files despite file picker allowing them

Bug Description
When using Claude Cowork to create a plugin, it loads the skill cowork-plugin-management:create-cowork-plugin. This skill gives the instruction to package the plugin as a ZIP using the .plugin extension, which will then be presented to the user to install into Claude Desktop. However, if you attempt to use the "Customize->Personal plugins->Upload plugin" menu to upload a .plugin file, it will fail with a generic error message "Upload failed. You can try again." I used Claude Code to analyze the plugin upload path in Claude Desktop's JS, and it found that the uploadPlugin handler has a check to where it only allows files with the .zip extension to be uploaded. Anything else fails with an error "Only .zip files are accepted." There are two bugs here: The first is that the backend is rejecting .plugin files even though the Upload Plugin file picker is filtered to show both .zip and .plugin. The second is that the more specific error message is not being passed back to the UI, causing a generic error message to show instead.

Environment Info

• Platform: win32
• Terminal: null
• Version: 2.1.86
• Feedback ID: 6c1564aa-598f-4dba-8774-77502ed12afe