Suggestion for custom file element

I am also investigating in a custom file element using JUpload, its a pretty cool upload applet with source code which can post to a php script to handle the uploads. I am looking into modifying the applet a little to do file extension validation and error detection either within the applet itself or from errors return from the php script to display to the end user. The applet source looks pretty clean and easy to understand. Lemme know your thoughts. http://jupload.sourceforge.net/