joomla 2.5组件中文件输入不工作

我有一个简单的组件，必须上传一个文件到服务器，但是当提交表单时，我看到这个错误:

Notice: Undefined index: uploaded_file in D:'wamp'www'joomla2.5'components'com_print'print.php on line 13

组件在前端，我的代码是:

<?php
// no direct access
defined('_JEXEC') or die;
jimport('joomla.application.component.controller'); 
?>
<form enctype="multipart/form-data" action="<?php $_SERVER['PHP_SELF']; ?>" method="post">
    <input type="hidden" name="MAX_FILE_SIZE" value="1000000" />
    Choose a file to upload: <input name="uploaded_file" type="file" />
    <input type="submit" value="Upload" />
</form>
<?php
echo $_POST['uploaded_file'];
echo $_POST['MAX_FILE_SIZE'];
?>

请帮。