System: Devuan Beowulf running Cinnamon using nemo as the file explorer

First install ufraw and libraw19 ( Not completely sure if this is needed but it is also used by shotwell and gimp so it's likely already installed )

# apt install ufraw ufraw-batch libraw19

Then create the thumbnailer file /usr/share/thumbnailers/ufraw.thumbnailer and place the following into the file (place other file types in the MimeType as needed):

[Thumbnailer Entry]
Exec=/usr/bin/ufraw-batch --silent --noexif --embedded-image --out-type=png --size=%s %u --overwrite --output=%o
MimeType=image/x-nef;image/x-nikon-nef;

Nemo should start showing the thumbnails for the RAW images now! Depending on the size of the RAW images the initial thumbnail creation may take some time.