[backend] support .sha256 format for static links in published area

Completly untested. I want to have links for the sha256 sum files. i.e. here: http://download.opensuse.org/repositories/Cloud:/Images/images/ for openSUSE-13.1-OS-guest.x86_64-raw.tar.bz2  should be a openSUSE-13.1-OS-guest.x86_64-raw.tar.bz2.sha256 file.
Is this the right approach?