I was having trouble getting image previews to work in nautilus.
Based on another post about broken image previews, I checked to see
whether eog was working.  Aha!  It wasn't loading the images either.
Checking ~/.recently-used  reveals that gnome thought these images
all had mime-type 'application/octet-stream' -- meaning that mime
type detection was screwed.

Since I am using debian,
 # apt-get install --reinstall shared-mime-info
was sufficient to fix the problem!

This seemed highly (er) non-intuitive, however, so I thought I'd
post it here, in case other debian users stumble across the same
problem and are interested in a quick easy fix.