thumbnailer: Fix (nearly) empty MimeTypes in thumbnailer
thumbnailer: Fix (nearly) empty MimeTypes in thumbnailer

Because we weren't waiting for loaders.cache to be created to run our
helper, and we weren't applying the environment variables to the correct
helper, we would end up with a broken list of supported MimeTypes.

https://bugzilla.gnome.org/show_bug.cgi?id=778451

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf