You can edit the widget in widgets.php which resides in the userfiles folder. User Files users the userfile_category table to hold the categories. Since the plugin allows for FTP upload each file needs to checked against the database to see if it is assigned to a category if you call the default category. If you have any other questions please let me know. Feel free to contact me directly by email here for a faster response.
These features are planned for a future release. The User Files plugin is in the process of a make-over. I am releasing small bug fixes to keep this one going until I have completed the big update. There is no release date set yet but it will include listing by categories in widget, page, and dashboard widget, pagination, listing by category, possibly drag-and-drop upload, multi-file upload, and more.