How can I skip "what should firefox do with this file" dialog on .mkv files

Hi I have been downloading a lot of .mkv recently and the dialog for "what should firefox do with this file" is popping up. I have been into the application tab in settings and could not find any application set to automatically play .mkv. When prompted to choose what to do with this file it says "VLC.mkv (default)" but looking through applications tab vlc has no .mkv setting. I just want to automatically download to a folder I choose I do not want it to ask me what i want to do with this file I want it to automatically select the save checkbox right away. Can anyone help?