/usr/share/doc/linux-image-extra-4.13.0-25-generic/changelog.Debian.gz is in linux-image-extra-4.13.0-25-generic 4.13.0-25.29~16.04.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/linux-image-extra-4.13.0-25-generic/../linux-image-4.13.0-25-generic/changelog.Debian.gz