How do I make a new action which is only available for certain file type?

I would like to add an option to the pop-up menu which shows up when I right-click on a file in the Project tree, but only for certain type, e.g. only file with extension .foo. I understand that I have to create a new action and register it in my plugin.xml but how do I limit it only to a certain types of files? Would appreciate it if anyone can point me to a more detailed example.