Desktop app sends JPEG images with image/png media type causing API 400 error

Bug Description

When attaching a JPEG image via the "+" button in Claude Code desktop app, the API returns a 400 error:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.2.content.0.image.source.base64: The image was specified using the image/png media type, but the image appears to be a image/jpeg image"},"request_id":"req_011CZ8KCwCosjscuCSJP3bRY"}

Steps to Reproduce

1. Open Claude Code desktop app
2. Click the "+" button to attach an image
3. Select a JPEG file (.jpg/.jpeg)
4. Send the message

Expected Behavior

The app should detect the actual image format (JPEG, PNG, WebP, GIF) from the file extension or magic bytes and set the correct media_type in the API request.

Actual Behavior

The app always sends media_type: "image/png" regardless of the actual image format. The Anthropic API validates the magic bytes against the declared media type and rejects the request with a 400 error.

Workaround

• Paste images from clipboard (Cmd+V) instead of using the "+" button
• Convert JPEG to PNG before attaching

Environment

• macOS Darwin 25.3.0 (Apple Silicon)
• Claude Code desktop app