/usr/share/doc/libwebpmux3/changelog.Debian.gz is in libwebpmux3 0.6.1-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libwebpmux3/../libwebp6/changelog.Debian.gz
/usr/share/doc/libwebpmux3/../libwebp6/changelog.Debian.gz