Sorry I forget to answere regarding the filter. It should work also. You could filter the files such in every other program. Please test:

"*.txt", "*.txt; *.cpp" or "bla*.*"

As you see, you could define more than one extension. Therefore you should only seperate with an semicolon. Maybe I have to describe that in a help text. Don and I tought about a own side for my plugins on Notepad++ homepage.

BTW: If you want to filter it correct, type in the filter and press enter to take over the new. It will be listed in the combo box.

Best Regards
Jens