convert a screencast
If you record screen with
(ctrl+shift+alt+R) in Ubuntu, you'll get a webm file, GitHub wants mp4. So you need to convert from webm to mp4.
ffmpeg command allows such a conversion.
Ubuntu's file navigator allows to have some right-click scripts shortcuts.
To be dropped and
chmod +x in something like
~/.local/share/nautilus/scripts/ffmpeg convert to mp4