To add a or indexing an extension to the n900 can use this command
/usr/bin/tracker-register-mimetype -c Videos -a video/x-matroska
This change is harmless, if it causes trouble you can remove it:
/usr/bin/tracker-register-mimetype -c Videos -d video/x-matroska
Of course you have to do this using the root shell.
I've tried it but the problem is that I get my music and video files can not be found, so I run the command to 2 to fix it.