No progress during upload

[Piotr Jasiun]

When a file is dropped into the editor, progress notification show 0% and then jump to the 100%. It might be because browsers do not sent onprogress event but I see other libraries do it better. Maybe it is because we use xhr.onprogress and maybe we should use xhr2.upload.onprogress. Maybe the order of adding listeners is wrong (now they are added before xhr.open what seems to be correct but maybe it is not). Maybe it is because we show only progress of uploading, not loading, but in fact it is loading what is the most time consuming. Or maybe it simple a bug. This should be investigated.