Avoid an undefined index notice in wp_handle_upload().

props tivnet.
fixes #27225.
Built from https://develop.svn.wordpress.org/trunk@27319


git-svn-id: http://core.svn.wordpress.org/trunk@27171 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Some playlist cleanup:

* Check properties against the `window` object when using `_.isUndefined()` on globals
* Fix a typo for `$safe_type` introduced in [27311]

See #26631.


Built from https://develop.svn.wordpress.org/trunk@27320


git-svn-id: http://core.svn.wordpress.org/trunk@27172 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Restore / re-tools some docs after [27313].

See #26631.


Built from https://develop.svn.wordpress.org/trunk@27321


git-svn-id: http://core.svn.wordpress.org/trunk@27173 1a063a9b-81f0-0310-95a4-ce76da25c4cd