I mean which file is open with which application is written somewhere in the system, I can't add it in deskypplet. I could, if the devs of eg TXPad,PyGTKEditor... added the possibility for that (not sure about this though)
Does that mean, you want a button? I can add it, it's not much work and easy to remove. Will take a while for favorites list anyway.
This would mean a lot of work (if there is no built in sollution for scrolled windows). It's easier for the user to get over it.