How should the backend verify that a file was uploaded successfully?

How should the backend verify that a file was uploaded successfully?

The backend should verify the expected storage object rather than accepting a browser's success claim. A completed transfer and an accepted gallery image are different states. The server needs evidence that the correct object exists and meets the application's requirements.

Start from the server's upload record

Look up the upload using a server-issued identifier and recheck the current user's permission. Obtain the bucket, key, expected size, and wedding association from that record instead of trusting a new client-supplied location.

Request object metadata through the supported storage API. The S3 HeadObject operation, supported by R2, retrieves metadata without downloading the body. Verify existence and actual byte size, handling failures without marking the item ready.

Validate content before making it viewable

A stored Content-Type can reflect what the uploader claimed. It does not prove the file is a safe, decodable image. Apply trusted content checks, image decoding, and processing limits appropriate to the allowed formats. OWASP's upload guidance explains these validation concerns.

Keep unvalidated uploads private or quarantined. If signed upload permission could still overwrite the object, finalize an immutable processed copy or otherwise prevent a later replacement from bypassing validation.

Make finalization repeatable and recoverable

Update the database status only after the required checks pass. Repeating the completion request should return the existing result rather than create another photo record.

Record rejected and abandoned uploads for cleanup. Handle an object disappearing during processing and a database write failing after storage succeeds. Test a forged completion request, incorrect size, invalid image content, and a valid upload retried after a network interruption. These checks connect storage evidence to a reliable, authorized gallery item.