File Upload fails in some times saying "Use getStringData() instead".
Reason for that, is that you use the Mime-Type send by the client, which could be wrong.
For further explanation see:
http://techblog.procurios.nl/k/news/view/15872/14863/mimetype-corruption-in-firefox.html

So even if the user upload the correct file, due to an error in his browser, ZK will also detect a wrong file type.
And converting back string data to binary data is a bit tricky due to charset issues.