post
https://api.delivr.to/payload/confirm-upload
Confirm that a file has been uploaded to S3 and trigger processing. Step 3 of 3 in the optimized upload flow. Automatically enforces delivery method restrictions: files >30MB are restricted to 'link' delivery only, files ≤30MB support both 'link' and 'attachment' delivery methods. This endpoint validates the upload, processes the file, and creates the payload record.