/usr/lib/x86_64-linux-gnu/libmng.so is in libmng-dev 1.0.10+dfsg-3.1+b5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmng.so.1
/usr/lib/x86_64-linux-gnu/libmng.so.1