This is done by using the "Select by name" menu item, which resides in the "Select" menu on Warp 3; on Warp 4, this menu item can be found in the "View" menu.

You then get a dialog window where you may enter a file specification such as "*.txt" and select or deselect files according to this specification.

Per default, XFolder also defines Ctrl+S as a folder hotkey for this function.