Adding `ffmpeg` installation, so there is no error message (`This client isn't compatible with the media and the server isn't sending a compatible media format.`) while playing in web browser.
Referring to https://jellyfin.org/docs/general/administration/installing.html#ffmpeg-installation, and as we ere not on Debian or DEbian derivative, we can just install `ffmpeg` and not the release built specifically for Jellyfin.
Tested, works fine.