Registers the file row and returns a presigned S3 URL that the client must then PUT the bytes to directly. Use this when your network can reach S3; if it is blocked by a proxy or CASB, use POST /users/file/upload instead, which streams through the API. The API applies no size limit here — the cap is whatever S3 accepts. Uploading does not by itself attach the file to a profile field: take the returned id and send it as the value of the field key via POST /users/{id}/data.