Download one artifact

The file's bytes, streamed, with the Content-Type it was saved with, Content-Disposition: inline and its filename. Send the key, or open the artifact's url from the session read, which carries expires and signature and needs no key until url_expires_at; HEAD on the link answers the headers alone. A link opens only the file it was issued for; a wrong, expired or altered link answers 404, as a missing file does.