sudo apt-get install gnome-raw-thumbnailer ufraw-batch
Try now if everything works, and your thumbnails show up. If not, try the following part.
In case the default config does not work for you, try this:
Edit the file /usr/share/thumbnailers/raw.thumbnailer
as root and insert the following content:
[Thumbnailer Entry]
TryExec=ufraw-batch
Exec=ufraw-batch --silent --size %s --out-type=png --noexif --output=%o --overwrite --embedded-image %i
MimeType=image/x-canon-cr2;image/x-canon-crw;image/x-minolta-mrw;image/x-nikon-nef;image/x-pentax-pef;image/x-panasonic-raw2;image/x-samsung-srw;image/x-olympus-orf;
Check if the mime type of your RAW images in in the MimeType
list.
After that, stop nautlius and clear the thumbnail caches:
nautilus -q
rm -rf .cache/thumbnails/* .thumbnails/*
Doesn't seem to work with olympus .orf files. Any ideas? I've tried both the gnome-raw-thumbnailer and ufraw-batch thumbnailers. The gnome-raw-thumbnailer generates black previews and the ufraw-batch does not appear to generate any previews.
Also see this (another person has the same problem): https://ubuntuforums.org/showthread.php?t=2392682