This answer is relevant to API calls using REST with Strapi V4.
Unlike other collections, users-permissions does not allow you to upload files during creation (like in the register call). To get this to work, you can wait until the register POST call returns, then make a second call using upload with the instructions found here: upload files related to an entry
Another way (ugly) would be to override the register call for the users-permissions plugin and do the upload in code yourself.