Comment 7 for bug 198154

He meant to attach a sample image to the bug, one that was causing the problem. But I think it may be something simpler. You should check ~/.local/share/mime/ and make sure you don't have an entries overriding JPG's MIME entry. The likely culprit is in ~/.local/share/mime/globs, a line something like the following might have crept in:
application/x-extension-jpg:*.jpg
If so, comment it out by adding "# " in front of it (note the space).
# application/x-extension-jpg:*.jpg

Nautilus's thumbnailer is looking for images with specific content types (mime-types), and application/x-extension-jpg isn't on the list (as it should be reporting image/jpeg for a jpg image). Certain applications (F-Spot in particular) have been known to override this entry in the past for whatever reasons.