I am having a lot of trouble with gthumb and the .nef raw image files
from my wife's Nikon DSLR. In spite of making sure that
seemingly-appropriate mime entries for .nef are in the system (thunar
recognizes .nef as "Nikon raw image", and thumbnails it), gthumb
--import-photos reports "no images found," gphoto2 refers to the .NEF
files as "application/x-unknown", and gthumb will only display the
low-res embedded thumbnails of the .nef files.

In contrast, on my wife's Ubuntu machine, gthumb will import and display
the .nef files just fine.

What am I doing wrong here?

-Andy