This endpoint returns mediaId that is the reference to the file and the upload URL. In order to upload your file, make a PUT request to the returned URL, with your file as payload.

Think of the alias part of MEDIA::ALIAS annotation as the reference to the placeholder in the Script. You link this placeholder to the actual file by using mediaFiles field in POST /mastering request, e.g. mediaFiles={"alias": "f22e7250"}

Click Try It! to start a request and see the response here!