oh I didn't know that. I read somewhere to do apt-get install gstreamer0.10-blah blah so I thought it's something I installed as add-on.

Anyway it also says to do:
/usr/bin/tracker-register-mimetype -c Videos -a video/x-matroska

but when I issue that command it says there's no such file tracker-register-mimetype. What is that supposed to do?