/usr/lib/libnautilus-extension.so is in libnautilus-extension-dev 1:3.10.1-0ubuntu8.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libnautilus-extension.so.1.4.0
/usr/lib/libnautilus-extension.so.1.4.0