Don't use wp_send_json_* handlers in the upload-attachment XHR handler.

The difference is the content type: application/json (which jQuery deserializes automatically for us) and the default text/html.

jQuery correctly handles application/json requests for IE, so we can continue to use the wp_send_json_* handlers elsewhere. Plupload rolls its own requests and does not handle application/json correctly. So, keep the standard text/html content type on upload-attachment.

props koopersmith.
see #22446.



git-svn-id: https://core.svn.wordpress.org/trunk@22845 1a063a9b-81f0-0310-95a4-ce76da25c4cd