Finish a direct upload once the file is sent: checks and keeps it.

Answers the upload's id and its status. The file is checked after this call answers, so a first confirm answers pending; the upload can be attached to a part revision (POST /parts/{part_id}/revisions/{revision_id}/datasets) once it is confirmed. Confirming again is safe and answers the current status. A confirm before the file has arrived answers 409 upload_not_received (send the file the way presign's method says, then confirm again); an upload that was refused, could not be checked or was cancelled answers 409 upload_rejected, upload_failed or upload_cancelled.