Tighten allowed upload file types.

Built from https://develop.svn.wordpress.org/trunk@25317


git-svn-id: http://core.svn.wordpress.org/trunk@25279 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Validate referrers to prevent off-domain redirects.

Built from https://develop.svn.wordpress.org/trunk@25318


git-svn-id: http://core.svn.wordpress.org/trunk@25280 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Better protocol validation in set_url_scheme().

Built from https://develop.svn.wordpress.org/trunk@25319


git-svn-id: http://core.svn.wordpress.org/trunk@25281 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Loose validation for is_serialized() in maybe_serialize().

Built from https://develop.svn.wordpress.org/trunk@25320


git-svn-id: http://core.svn.wordpress.org/trunk@25282 1a063a9b-81f0-0310-95a4-ce76da25c4cd