My Extensions is not in the list over supported extensions

I have a bounce of file with the extension .pro6. They are not showing up when I chose the containing folder. If I rename the extension to something else matching the extension list, it will be shown.

So now my question!
Is it possible to add my extension to the list? I have checked you code, but can’t find anywhere where you are creating the extension list. So any hint will be nice.

Thanks for a good product, It seams better than the Media From FTP plugin, you made to. That I have used on several websites, but will move on with this updated version.