/usr/lib/x86_64-linux-gnu/libXdmf.so is in libxdmf-dev 2.1.dfsg.1-6build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libXdmf.so.2.1
/usr/lib/x86_64-linux-gnu/libXdmf.so.2.1