Thank you for your fast reply and the sample. It turned out that the issue is because of a cross origin request (the endpoint for the image upload is on another domain). JavaScript, however, can only access a couple of headers in that case. Thats why my name header is missing. I can try to work around it by abusing the content-type header for the image name. This is ugly but might work.
It would be good if you can make the response body accessible through the ImageSuccessEventArgs.Response object and not only the headers. Is that something you can do for a future update?