Comment 12 for bug 1768855

Romano,

"Unfortunately, still I can't generate thumbnails for raw files..." - to workaround this I installed old ufraw-batch package from ubuntu 16.04 - https://ubuntu.pkgs.org/16.04/ubuntu-universe-amd64/ufraw-batch_0.20-3build1_amd64.deb.html - also needs liblensfun0 and libpng12-0 - listed in Requires.
Download debs, install using dpkg -i, and mark packages hold to prevent update.