Aborting upload doesn't really abort

[Artur Delura]

To reproduce this issue, you have to have properly setup CKFinder with CKEditor. Also you should add some sleep in connector to be available to abort uploading before it's done by server.

1. Open sample ​http://ckeditor.dev/plugins/uploadwidget/dev/upload.html
2. D&D some image to initiate uploading.
3. Delete an image before uploading is done.

There should be an information visible that uploading was aborted by the user.

Unexpected Result: image has been physically uploaded to the server.

What I found is that abort in XHR is called synchromously when progress comes from the server. If file is relatively small, then there is no progress called until file is fully loaded.