I had the same issue myself just now and got it fixed like so:

In /usr/share/applications/defaults.list and /usr/share/applications/mimeinfo.cache, change the entry
video/x-msvideo=hildon-mp_ui.desktop
to..
video/x-msvideo=hildon-kmplayer.desktop

This will set kmplayer to open ".avi" files by default. There are several other file extensions listed that you can change.

For a complete overview of file extensions check out /usr/share/mime/globs

/usr/share/applications/hildon/* will give you an idea which applications you have to work with.

Cheers