Quick tip: You can get XCF thumbnails in Nautilus by using GNOME XCF Thumbnailer. However, the tool needs a fix to work with the latest Nautilus 3.x. This should also work with Nemo, the default Cinnamon file manager.
XCF (eXperimental Computing Facility) is the native image format used by GIMP image editor.
To get XCF thumbnails in Nautilus, firstly install gnome-xcf-thumbnailer. In Ubuntu, use the following command:
sudo apt-get install gnome-xcf-thumbnailer
At this point the the thumbnails don't work yet because GNOME XCF Thumbnailer doesn't install a .thumbnailer file, required by Nautilus 3.x. To fix this, use the following commands:
sudo mkdir -p /usr/share/thumbnailers/ echo -e "[Thumbnailer Entry]\nTryExec=gnome-xcf-thumbnailer\nExec=gnome-xcf-thumbnailer %i %o\nMimeType=image/x-xcf;image/x-compressed-xcf;" | sudo tee /usr/share/thumbnailers/gnome-xcf.thumbnailer
New XCF files (or if you move some already existing XCF files) should have thumbnails now but existing ones may not get thumbnails yet. Let's fix this too by removing the failed thumbnails from cache:
rm -r ~/.thumbnails/fail rm -r ~/.cache/thumbnails/fail
That's it. Nautilus should now display XCF file thumbnails, as you can see in my screenshot above. I've tested this with Nautilus 3.4, 3.6 and 3.8.