/usr/lib/x86_64-linux-gnu/libwebpmux.so is in libwebp-dev 0.4.0-4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libwebpmux.so.1.0.0
/usr/lib/x86_64-linux-gnu/libwebpmux.so.1.0.0