Ignore user ID post data. Merges [25316] to 3.6.

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Tighten allowed upload file types. Merges [25317] to 3.6.

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Validate referrers to prevent off-domain redirects. Merges [25318] to 3.6.

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Better protocol validation in set_url_scheme(). Merges [25319] to 3.6.

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Loose validation for is_serialized() in maybe_serialize(). Merges [25320] to 3.6.

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3.6.1-RC1

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