You seem to have reinvented javax.swing.filechooser.FileNameExtensionFilter
Setting location-sensitive tool tips on a JFileChooser can be tricky. You can obtain a reference to the JList subclass that displays the folder contents, and do some fancy stuff with locationToIndex(...) but what do you do when the user toggles the display between a list and a table? Have you thought about that?
There are no new questions, but there may be new answers.