Support JSON response in Internet Explorer < 10

Internet Explorer older than 10 does not accept
Content-Type: application/json
and would start to download response.

Also header Content-Disposition
would force IE to download response.

It would only occur if you would try to upload file by ajax in IE < 10, 
because IE 7 - 9 do not support uploading file with XHR.

To test it see a test module in Code Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=32285

Related PR which are already outdated and closed:
https://github.com/joomla/joomla-cms/pull/2013
https://github.com/joomla/joomla-cms/pull/2166