Filemtime seems to return the date of the EARLIEST modified file inside a folder, so this is a recursive function to return the date of the LAST (most recently) modified file inside a folder.
<?php
$allowedExtensions = array(
  'zip',
  'rar',
  'pdf',
  'txt'
);
function filemtime_r($path)
{
    global $allowedExtensions;
    
    if (!file_exists($path))
        return 0;
    
    $extension = end(explode(".", $path));     
    if (is_file($path) && in_array($extension, $allowedExtensions))
        return filemtime($path);
    $ret = 0;
    
     foreach (glob($path."/*") as $fn)
     {
        if (filemtime_r($fn) > $ret)
            $ret = filemtime_r($fn);    
            }
    return $ret;    
}
?>