multimedia_id is a unique identifier for specific media files. As you can see here (https://github.com/dimagi/commcare-hq/blob/fb9b77e26b25762dc8bf3fef42ad5588f7baf59c/corehq/apps/hqmedia/models.py#L203), media files are retrieved from couch by the hash of the file  (`get_by_data` does the same) , and new media files are only made when couch could not find an existing copy of the uploaded file in the DB. So media_ids are unique AND don't get changed when you reupload the same file to couch