The file field, of course, doesn't have good UI. I will search to see if the WordPress media can force users to select files from the computer. If there's a solution for that, I will add to the file_advanced field.
But wait... the UI is OK in my case in fact.
The problem is the thumnnnail preview and edit/delete links which are not working when file type is in a group.
(but ok when using it with "classic" fields)