Thumbnailers for Krita (.kra), Gimp (.xcf), OpenRaster (.ora), WebP (.webp) and Adobe Photoshop (.psd) files
19.04.2015 [Software]
If you want thumbnails in nautilus (the file manager, that is part of GNOME and used in Ubuntu by default, nowadays it's just called "Files"), nemo (cinnamon I think) or pcmanfs (xfce) for one of the formats mentioned in the title, it's actually quite simple to add them.
Nautilus and some other file managers run scripts and executables to create their thumbnails, and the associations are controlled through files located at /usr/share/thumbnailers/ in your usual Linux distribution.

Simply create the files below inside said directory, clean the cache (rm -rf ~/.cache/thumbnails) and restart your file manager. For nautilus for example, you can run "nautilus -q" (-q for "quit") in a terminal to terminate all running instances of it.
Make sure that imagemagick is installed for psd and webp and xcftools for xcf. If the webp and psd thumbnailers still fail to work, you might have to work around a bwrap issue that is present in some distributions (including Debian/Ubuntu). Please refer to this article in that case.

If you don't want to create the files manually, you can download this small script that can create them for you.

File: /usr/share/thumbnailers/kra.thumbnailer
[Thumbnailer Entry]
Exec=sh -c "unzip -p \"%i\" preview.png > %o"
File: /usr/share/thumbnailers/ora.thumbnailer
[Thumbnailer Entry]
Exec=sh -c "unzip -p \"%i\" Thumbnails/thumbnail.png > %o"
File: /usr/share/thumbnailers/webp.thumbnailer
[Thumbnailer Entry]
Exec=convert "webp:%i" -scale 512x%s png:%o
File: /usr/share/thumbnailers/psd.thumbnailer
[Thumbnailer Entry]
Exec=convert "psd:%i[0]" -scale 512x%s png:%o
File: /usr/share/thumbnailers/xcf.thumbnailer
[Thumbnailer Entry]
Exec=sh -c "xcf2png \"%i\" -o - | convert -scale 512x%s - png:%o"