By default Nautilus does not create thumbnails for RAW images. Installing
raw-thumbnailer should help. For Nikon RAW images (.NEF) this didn’t work for me initially. But there is a simple workaround:
/usr/share/thumbnailers/ create a file nef-thumbnailer.thumbnailer with the following content:
[Thumbnailer Entry] Exec=/usr/bin/gnome-raw-thumbnailer -s %s %i %o MimeType=image/x-nikon-nef;
The content however is old and might be outdated.