/usr/lib/x86_64-linux-gnu/libmicrohttpd.so is in libmicrohttpd-dev 0.9.51-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmicrohttpd.so.12.39.0
/usr/lib/x86_64-linux-gnu/libmicrohttpd.so.12.39.0