如何使用ajax将上传的文件名提取到控制器？

Question

html代码：

    <form action="" method="POST" role="form" id="registration_form" enctype="multipart/form-data">

<label> File Input: </label>

<input type="file" name="userfile" id="userfile"> 

<input type="submit" name="submit" value="Upload" />

</form>

查看页面上的Js： $：

    <script src="../../js/jquery-validation.js"></script>
<script> 
$(document).ready(function() { 

$("#registration_form").validate({ submitHandler: submitForm });

function submitForm() { 
var photo = $('#userfile').val();
alert(photo); 
$.ajax({ 
url: "<?php echo base_url();?>upload/do_upload",
type: "POST",
data: { user_photo: photo }, 
success: function (data) { alert(data); } 
});
} 
});
</script>

控制器代码：

       <?php 
class Upload extends CI_Controller {
function __construct(){
parent::__construct();
$this->load->model('registration_model');
} 
public function index(){
$this->load->view('upload_view');
} 
public function do_upload() { 
$config = array( 'upload_path' => "/var/www/html/upload/", 'allowed_types' => "gif|jpg|png|jpeg|pdf|doc|docx"); 
$this->load->library('upload', $config);
if(!$this->upload->do_upload('userfile')){ 
$error = array('error' => $this->upload->display_errors()); 
print_r( $error); 
} else{
$data = array('upload_data' => $this->upload->data());print_r($data);
} 
}
}
?>