I know how to validate this client and server side by extension and MIME type. That's fine, implemented, ready to go.
What I want to do is in the file upload dialog there is the little drop down list that by default shows "All Files." I have seen similar controls in other sites and programs that effectively sort the directory for the user by the different options in the drop down ( just music files or just images, sometimes with the "All Files" option, sometimes not) and limit the user from seeing any file that isn't in the list.
How do I do that?