Upload a file for embedding in docs, data entries, or insights. The request must use multipart/form-data encoding with a single file field containing the file data.

After uploading, use the returned file ID to reference the file in document content (e.g. as a cover image via cover_image_file_id on the Create doc or Patch doc endpoints).

Limits: Maximum file size is 100 MB. Orphaned files that are not embedded in any document within 24 hours are automatically cleaned up.

Returns the file metadata including its processing status.