简单的文件下载程序

我是php的新手，通过阅读SO中列出的一些问题，我成功地将这个简单的下载脚本串在了一起，我想请那些更熟悉php的人看看下面的代码，看看我的实现是否有任何明显的缺陷或应该更改的内容。

只是让你知道，在我有限的测试期间，一切似乎都很好，但正如我所说，我对php还很陌生，我想确保我不会错过一些可能会在以后破坏脚本的东西。

<?php
//Settings
$filesPath   = './files';
$fileName    = $_GET['file'];
$allowedExts = array('jpg','png','gif');

//Functions 
//Returns the extension portion of a filename.
function file_extension($fileName)
{
    $path_info = pathinfo($fileName);
    return strtolower($path_info['extension']);
}
//Validation and processing
//Check that a file is actually being requested
if (empty($fileName)) {
    die('no file was requested');
}
//Check that the file is allowed to be downloaded   
if (!in_array(file_extension($fileName), $allowedExts)) {
    die('you cannot download this file');
}
//Get the file  
if (file_exists($filesPath . DIRECTORY_SEPARATOR . $fileName)) {
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename=' . basename($fileName));
    header('Content-Transfer-Encoding: binary');
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    header('Pragma: public');
    header('Content-Length: ' . filesize($fileName));
    ob_clean();
    flush();
    readfile($fileName);
    exit;
}
?>

TIA，Dave