Gnome’s file manager Nautilus doesn’t display video thumbnails under Ubuntu 24.04. Thankfully this solution still works: https://askubuntu.com/questions/1034595/thumbnails-not-showing-in-video-in-ubuntu-18-04
sudo apt install ffmpegthumbnailer
sudo apt install gstreamer1.0-libav
rm -r ~/.cache/thumbnails/fail
Reboot
Done!
George's Techblog 