When the @RequestBody is added, Spring would require the content type to be application/json and the JSON body is required.
When the @RequestPart is added, Spring would require the content type to be multipart/form-data and input as form
=> If both of them are included in a controller, the error 403 will be thrown because the content type is not correct.

In this case, I think we can input username and email as @RequestPart
@RequestPart("username") String username