thumbnailers troubleshooting

["Kotrla Vitezslav" <kotrla ceb cz>]

I'm using Nautilus 2.6 (Debian/unstable) at the moment and in  the course 
of all previous upgrades Nautilus stopped thumbnailing .avi, .mpg, .ogm and
likes of them.

Could someone more knowledgeable than this person please post a kind
of checklist I could go through to verify Nautilus' thumbnailing capabilities and
settings?

It would be nice to have Nautilus thumbnailing videos again. Thanks for any tips,
hints or even the longed-for checklist :-)

Vit