ajaxForm不回呼

I know that this is already covered by a lot of you and in hundreds of posts but I didn't see anyone with a problem like I'm having.

I'm trying to implement the AjaxForm to upload files. I read all the tutorials, documentation, examples ... everything.

HTML:

<form action="scripts/upload_script.php" method='post' id="upload_picture_form" enctype="multipart/form-data">
    File: <input type="file" name="upload" id="upload" /></p>
    <p><input type="submit" name='submit' value="Submit" /></p>
</form>

Jquery:

$('#upload_picture_form').ajaxForm({
  success: function(){
    alert('success');
  },
  error: function(){
    alert('error');
  },
  resetForm:true
}); 

When I submit the form, the ajaxForm is called, on fireBug I can see the php file been called and I can also see that the response is '200 OK' after a few 'ms' and I can see the response in HTML sent by the php.

The problem is after the response no 'success' or 'error' is triggered, the form is no reseted ... looks like there is no callback.

Can anyone tell me what I'm doing wrong? Does the 'echo' from the php has to be in some specific format? I'm sending a simple "File: image.jpg (7487 bytes)" back to the jQuery.

Regards.

here is the small example of ajaxFrom that return the callback event as well:

<form id="imageform" enctype="multipart/form-data" action="get_img.php" method="post">
 <tr>
   <td valign="top"><label>Select File</label> :</td>
   <td><input type="file" name="myfile" id="myfile" /> <br />
   <em>.jpeg, .jpg, .gif, .png, .bmp</em>
   </td>
 </tr>
</form>

<div id="result"></div>

now working with jquery :

$(function(){
$('#myfile').live({
    change: function(){
            $('#result').ajaxStart(function(){
                $(this).html('<div class="loader" />');
            });

            $('#imageform').ajaxForm({
                                 target: '#result', 
                                 success: function(){ // Call Back Function } 
                            }).submit();
        }
});
});

the purpose of implementing the ajaxFrom here to submit the image without click event, and in the above jquery codes, i have attached the event on Change, the form is submitted automatically and call the get_img.php which is in action.

on the target you get what you have returned through the file get_img.php,
and on success you set any callback, method that you want.