The HTML4 Plupload handler uses a hidden iframe to POST the upload form,
Unfortunately Internet Explorer 9 doesn't support the `application/json`
content-type which `wp_send_json_success()` and requires `text/html` instead.
This partially reverts [30354], keeping the better error messages.
Fixes#31037 for trunk.
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git-svn-id: http://core.svn.wordpress.org/trunk@31410 1a063a9b-81f0-0310-95a4-ce76da25c4cd