/usr/lib/libmathview_frontend_custom_reader.so is in libgtkmathview-dev 0.8.0-7.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libmathview_frontend_custom_reader.so.0.8.0
/usr/lib/libmathview_frontend_custom_reader.so.0.8.0