/usr/lib/x86_64-linux-gnu/libavformat.so is in libavformat-dev 6:11.12-1~deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libavformat.so.56.1.0
/usr/lib/x86_64-linux-gnu/libavformat.so.56.1.0