The CB Team has just released version 1.1 of its CB Ajax Filefield plugin. It is available for all CB Documentation subscribers and can be quickly downloaded from their Doc-Subs tab on their Joomlapolis profile.
This plugin will enable the creation of CB file fields that can be added to user profiles and allow profile owners to upload a file in such a CB field. Each CB field can be configured to accept certain file extensions and reject anything else. You can see it in action on our CBSubs demosite where each profile has an ajax file field in its user status section.