sudo apt-get install ffmpegthumbnailer
cd /usr/share/thumbnailers/
sudo cp totem.thumbnailer totem.thumbnailer.original
sudo vi totem.thumbnailer

#replace line 2-3 with:
TryExec=/usr/bin/ffmpegthumbnailer
Exec=/usr/bin/ffmpegthumbnailer -s %s -i %i -o %o -c png -f

#exit nemo
nemo -q