Check the following setting: Nautilus > Edit > Preferences > Preview > Only for files smaller than
10MB seems to be a default value here. At least i cant remember i have changed it myself.
Then ensure you have either gnome-raw-thumbnailer installed. Just type in console
gnome-raw-thumbnailerInstall it if ubuntu suggests it with
sudo apt-get install gnome-raw-thumbnailer
Next ensure you have now a file
/usr/share/thumbnailers/gnome-raw.thumbnailer
Ensure the file contains mime for RAW2 images
image/x-panasonic-raw2
On my PC it didn't work at this point. So i have looked to the
/usr/share/mime/packages/gnome-raw-thumbnailer.xml
Here was RW2 file extension missing. Add the following line to this xml. Case matters. Correct it if your cam saves photos in lower case.
<glob pattern="*.RW2" />
And i can preview them now like any other pictures without to have open each single one. Yeeee!