Original bug title:
UploadWizard should not use iframe transport for upload_by_url requests to the wiki-own-API (FlickrUploading)
Follow-up Bug 65406
There is no reason why UploadWizard uses a complicated iframe technique to send a simple XHR to the API. The XHR has no file attached so, disregarding abstraction, $.post() would do the job in all browsers.