- Remove unreliable URL pass-through strategy (image_url field)
- Always download and upload images as File objects
- Get MIME type from HTTP response headers for URLs
- Use File constructor (not just Blob) for proper multipart metadata
- Add comprehensive error logging with headers and file metadata
- Simplify to single reliable upload path
Fixes 400 'Upload a valid image' error caused by Blob not providing
proper filename/MIME metadata in multipart form data.
