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

See #26631.



git-svn-id: https://develop.svn.wordpress.org/trunk@27244 602fd350-edb4-49c9-b593-d223f7449a82
Get the proper image size for an item's thumb in playlist JSON.

See #26631.



git-svn-id: https://develop.svn.wordpress.org/trunk@27245 602fd350-edb4-49c9-b593-d223f7449a82