/usr/share/doc/kde-config-mailtransport/changelog.Debian.gz is in kde-config-mailtransport 15.12.3-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/kde-config-mailtransport/../libkf5mailtransport5/changelog.Debian.gz