What doesn’t work exactly? If you are trying to add a file to an existing user entity, then these instructions work: Upload file to existing entity.
For a users-permissions user the fields are:
“ref”: “plugin::users-permissions.user”
“refId”: <the id of the user record you want to attach the file to, as a string>
“field”: <the name of your field in the user collection, for example ‘avatar’>
Make a multipart POST request to the url: http://localhost:1337/api/upload with your bearer token.
Make sure that you have allowed the upload action in the Authenticated Role under the Upload plugin section.
If you are trying to upload a file during a user registration (something that I needed), then it won’t work as it does with other content-types using the “Upload file during entry creation” instructions. Instead you have to re-implement the register function and upload the file after the user record is added (copy a lot of code from the plugin-users-permissions plugin auth controller) into your strapi-server.js file. If you do that, be warned as the dev team has updated the register code recently and it broke my version of it until I updated it to match their code.
I hope this helps.