The thunar-thumbnailers project provides additional thumbnailers for
use by the Thunar file manager. Thumbnailers are little utilities that
run in the background to generate previews for certain file types.

dcraw is an optional dependency for raw image support.
If you wish to enable support for it, you will need to edit the
build script to do so - the required lines are commented out. If
you choose to compile with dcraw (--enable-raw), you'll also need
raw-thumbnailer.