php使用post方式获得文件扩展名

<form action="" method="post">    <input type="file" value="" name="file" size="20">    <input type="submit" value="check"></form><?php    header("Content-type:text/html;charse=utf8");    if(isset($_POST['file'])){        $pic = $_POST['file'];        $pics = explode(".", $pic);        echo "上传文件的扩展名为：".$pics[count($pics)-1];            }?>