sudo dnf install raw-thumbnailer
sudo dnf install ufraw
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:
nautlius -q
rm -rf .cache/thumbnails/* .thumbnails/*