Docs: Correct wp_ext2type() return type from void to null

[rollybueno]

Per the PHP manual, if a function omits return, the value null is returned.

wp_ext2type() either returns a string for known extensions or null when no match is found. The doc block is updated to reflect this: string|null.

Trac ticket: https://core.trac.wordpress.org/ticket/63166

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[shail-mehta]

LGTM 👍

[SergeyBiryukov]

Thanks for the PR!

In core, I think we generally use null in the @return tag when the function has an explicit return null line. Otherwise, we use void, which is the case here. So it seems like this change would be inconsistent with the rest of core.

That said, we could perhaps reconsider this, but ideally in a consistent way 🙂

[rollybueno]

Thanks. Closing here then, we can think about that sometime in the future.