This is a note to myself in case I need it again in the future...
pcmanfm, my favorite file manager, uses the gnome3 thumbnailing system, which makes it very easy to write thumbnailers for new filetypes. Thumbnailers are small programs that take a file of a certain type, and create a small png image of it. So in this example, I'm using imagemagick, the famous image conversion tool that you'll find on almost any linux distro, to make a thumbnailer for HDR images. The thumbnailer is composed of two files:
hdr.thumbnailer (place it in /usr/share/thumbnailers or ~/.local/share/thumbnailers):
Exec=imagemagick-thumbnailer %i %o %s
imagemagick-thumbnailer (place it in /usr/bin or any other folder that is in your binary search path):
/usr/bin/convert "$1" -thumbnail 64x64 -gravity center -background white -extent 48x48 png:"$2"
One last thing, I suggest you delete /usr/share/mime/packages/kde.xml (installed by kdelibs5-data) because it horribly messes with several file formats (such as HDR) and the mime system doesn't recognize them anymore.