[jQuery] No response with jQuery Form Plugin in Opera

[Uznick]

I have a form, that uploads a file. It is initialized by the following
code:

$(document).ready(function() {
$('#image_upload_button').click(function() {

            $('#imageform_step1').ajaxSubmit({
                beforeSubmit: function(formData, jqForm, options)
{ alert('sending'); },
                success: function(responseText, statusText)
{ alert(responseText); },
                dataType: 'html'
            });
        }
        return false;
    });
}

And the form is simple:

<form action="/blog/add/ajax_image/step1/" method="post"
enctype="multipart/form-data" id="imageform_step1">
<input name="original_image" id="original_image" type="file"/>
<a href="#" id="image_upload_button"><span>Upload image</span></a>
</form>

The problem is that in Opera from 9.27 up to the last available stable
version success event never comes.
In all other browsers (ranging from ie6 and up to firefox 3 and
safari) everything is ok.

Version of jQuery is 1.2.6, jQuery form plugin is 2.12.