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/*
I know this isn't the point of this post but none of the recommended solutions here seemed to work of Mint 19.1. It does seem like 19.1 mate has the .thumbnailer file that @ghalfacree mentioned. Haven't had a lot of luck with editing it, quitting caja, and removing the cache