Dion Hulse 281b8022e1 Prevent IE9 and lower displaying the download file dialogue when attempting
to upload using the `html4` Plupload handler.

The HTML4 Plupload handler uses a hidden iframe to POST the upload form, and
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.

Merges [31429] to the 4.1 branch.
Fixes #31037.

Built from https://develop.svn.wordpress.org/branches/4.1@31430


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