Supply mime-type instead of extension for an item's `type` property in playlist JSON.

See #26631.


Built from https://develop.svn.wordpress.org/trunk@27244


git-svn-id: http://core.svn.wordpress.org/trunk@27101 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Get the proper image size for an item's thumb in playlist JSON.

See #26631.


Built from https://develop.svn.wordpress.org/trunk@27245


git-svn-id: http://core.svn.wordpress.org/trunk@27102 1a063a9b-81f0-0310-95a4-ce76da25c4cd