Определение типа файла с помощью PHP. Что такое Magic Database?

Я пытаюсь определить, действительно ли некоторые файлы являются изображениями (с помощью PHP). I've been advised to use finfo and i'm trying to understand how it works.

What I don't get is - what is a magic numbers database and how does it work? I'm a bit puzzled - does each file have certain "magic number" that you compare against that database?

Also - I have it on my debian squeeze - but will it also be available on WIN platform? or would one have to attach that database along with the app?

<?php
$finfo = new finfo(FILEINFO_MIME, "/usr/share/misc/magic.mgc");
if (!$finfo) {
    echo "Opening fileinfo database failed";
    exit();
}

/* get mime-type for a specific file */
$filename = "/usr/local/something.txt";
echo $finfo->file($filename);

?>