Xcf is the native file format of the Gimp image editing program.
To get xcf thumbnails in Nautilus, the gnome-xcf-thumbnailer has to be installed. Then a custom thumbnailer is placed in /usr/share/thumbnailers. Open a terminal window and run:
wget -q http://goo.gl/qdhC7 -O- | sudo tee /usr/share/thumbnailers/xcf.thumbnailer
ReferencesGenerating Thumbnails for Nautilus using Gimp
XCF thumbnailer doesn't work