/usr/share/doc/ffmpeg-doc/changelog.Debian.gz is in ffmpeg-doc 4:0.8.1-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/ffmpeg-doc/../libav-doc/changelog.Debian.gz
/usr/share/doc/ffmpeg-doc/../libav-doc/changelog.Debian.gz