You can create whatever type of customized file-picker you want, just give it is different appearance name that you reference in your XSD.
Please note that with 2.2 Enterprise this December, we will also add top-level directory limiters to the file picker control, as well as the ability to list results from saved searches (for example, to allow selection of just XMLs created from a particular web form).
More information on the above can be found on our wiki. Cheers!
Kevin