Hi,
wenn ich das richtig verstanden habe mach einfach:
  
<form action=".." method="post" id="myform">  
...  
<input type="file" name="file" style="display: none;" onchange="submitMyForm()" />  
...  
  
</form>
  
<script type="text/javascript">  
function submitMyForm() {  
document.getElementById("myform").submit();  
}  
</script>