Question: Before I upgraded to Ubuntu 12.04, thumbnails on video files worked nicely, after the upgrade, thumbnails aren't generating on video files and only on video files, most of them are .mkv files. I have some .pdf files and they all have thumbnails. I have tried to clear the thumb cache, but no luck.

Answer: 
I followed the instructions in [this forum](http://ubuntuforums.org/showthread.php?t=1801173) and it worked perfectly.

Install libxine1-ffmpeg.  

`sudo apt-get install libxine1-ffmpeg`

Then to force nautilus to generate new thumbnails execute  

`rm -r ~/.thumbnails/fail  

rm -r ~/.thumbnails/normal  

killall -9 nautilus`

These commands will remove (delete) thumbnails from these folders.

Open nautilus again and you should be able to see thumbnails for all your videos.
I installed `ffmpegthumbnailer`:

```
sudo apt-get install gstreamer0.10-plugins-bad gstreamer0.10-plugins-base gstreamer0.10-plugins-good gstreamer0.10-plugins-ugly totem libxine1-ffmpeg

```

It works in 13.10