/usr/share/doc/libmono-system-net-http4.0-cil/changelog.Debian.gz is in libmono-system-net-http4.0-cil 3.2.8+dfsg-4ubuntu1.
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It is a symlink to /usr/share/doc/libmono-system-net-http4.0-cil/../libmono-corlib4.5-cil/changelog.Debian.gz