Ok you should use: 

<input type="button" id="submitbutton" value="submit" onclick="doStart();">

for the button, this way it works.

But I have a problem. Now if I don't want to upload picture in a form, the button triggers nothing (I have removed the return false;) because obviously the OnTaskComplete is not triggered.

How could I detect if the upload queue has a file and according to that trigger either the upload/submit or only submit? Is it possible?