/usr/lib/mono/compat-2.0/System.Web.Extensions.dll is in libmono-system-web2.0-cil 2.10.8.1-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/compat-2.0/../gac/System.Web.Extensions/1.0.61025.0__31bf3856ad364e35/System.Web.Extensions.dll