Submits form data with support for deferred file uploads.

Integration Flow:

1. Submit form data including file names for any file fields
2. If files need uploading, response includes presigned S3 URLs (status: accepted)
3. Upload files to S3 using the presigned URLs (multipart form POST)
4. Call POST /:token/confirm to verify uploads and complete submission

Response Status:

• completed: No files to upload, submission processed immediately
• accepted: Files need uploading, use provided presigned URLs then call /confirm

File Upload:
When uploading to S3, send a multipart form POST to the url with:

• All fields from the fields object as form fields
• The file content as file field