add more extension, .3gp .avi

I have been frustrated for years about having to download all mp4 videos sent to me via
Gmail before I could view them. Cluttered up my hard drive with useless videos.
This add-on solved the problem.
What a Relief!! I use Firefox and Linux Mint OS.

Thank you, thank you.

Does what it says

I added it, reopened Firefox and it did not work.