Upload Image with ajax or xajax

[mwenyenia07]

//CODE to handle uploads

    function do_upload($data)
    { 
                $objResponse = new xajaxResponse();
        $config['upload_path'] = './photos/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size'] = '100';
        $config['max_width']  = '1030';
        $config['max_height']  = '773';

        $this->load->library('upload', $config);


        if ( ! $this->upload->do_upload($data['userfile']))
        {
            $error = array('error' => $this->upload->display_errors());

            //$objResponse->Assign("ajax_div", "innerHTML",$this->upload->display_errors());
            $objResponse->Assign("ajax_div", "innerHTML",$this->upload->display_errors());
        }
        else
        {
            $data = array('upload_data' => $this->upload->data());

            $objResponse->Assign("ajax_div", "innerHTML", "Just saying hello from XAJAX.");
        }
       return $objResponse;
    }

//html to send image

<form id="up_form" enctype="multipart/form-data" name="up_form" method="post">

<input type="file" name="userfile" id="userfile" size="20" />

<br /><br />

<button type="button" onclick="xajax_do_upload(xajax.getFormValues('up_form'));">Save</button>
</form>

The above code is intended for uploading an image. I am using codeIgniter and Xajax framework but each time I upload, The system says 'You have not selected a file to upload.'

Can somebody help me.

Thanks.