Uploading files through media modal causes error in < IE10

[joehoyle]

Trying to upload a file in the Media Modal in IE9 and lower triggers a "File Download" popup because the "async-upload.php" endpoint now returns Content-Type: application/json since 4.1. IE9 and lower don't understand this content-type so trigger a file download dialogue.

This was discussed at some length with the introduction of the new Media Modal here: https://core.trac.wordpress.org/ticket/22446#comment:20 long story short - returning application/json is not usually a problem as jQuery automatically handles it for older versions of IE, however this particular request is handled by Plupload which does not handle this edge case.

Before [30354] this endpoint returned text/html as it's Content Type; however now wp_send_json_error and associated functions are used which triggers a Content-Type header of application/json.

I'm not sure what versions of IE is "officially" supported, but it would seem right now the upload file workflow feels broken in < IE10.

[dd32]

As @pento pointed out - r22845 from the linked thread above.

For 4.1.1, I think we should just revert those changes to admin-ajax.php made in [30354].

31037.diff​ implements an (untested) alternative for 4.2, simply add a $content_type parameter, so we're explicit in what we're sending so this doesn't happen again.. I'm not entirely sure I like the parameter, but since it's needed for older browsers, doesn't seem too horrible.

[dd32]

In 31429:

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,
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.

[dd32]

In 31430:

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.