/usr/share/isenkram/Fw-Contents-amd64-stretch-non-free is in isenkram-cli 0.22.
This file is owned by root:root, with mode 0o644.
It is a symlink to /usr/share/isenkram/./usr/share/isenkram/Fw-Contents-amd64-sid-non-free
/usr/share/isenkram/./usr/share/isenkram/Fw-Contents-amd64-sid-non-free