I am not sure if this feature is in Pro, but when you have backups that use %Y etc in the name, when it comes to the restore, Virtualmin expects a filename called "%Y". Wouldn't it be better to do list the files that are on the ftp (if a %Y or something was in the file name) and allowing the user to select the backup ?
Some "code" :
if (in_string ("%",ftp_filename))
Hope that is clear :)