I've been trying to upload a user profile picture during the user registration using Service 3 and so far I haven't had any luck. I tested passing a hard coded fid in the field "picture" and also tried to pass the fields "filemime", "filename", etc. and it didn't work neither. Any idea about what fields I need to populate?

I guess it must be related to this post Using Drupal Services and DIOS SDK for setting user picture in iOS app but it doesn't have and answer neither.