什么是“$ _FILES腐败攻击”

I came across the following piece of Code for validating uploaded files here: http://php.net/manual/en/features.file-upload.php

// Undefined | Multiple Files | $_FILES Corruption Attack
// If this request falls under any of them, treat it invalid.
if (
    !isset($_FILES['upfile']['error']) ||
    is_array($_FILES['upfile']['error'])
) {

But what is a "$_FILES Corruption Attack"?
(How) does the mentioned Code prevent it?
How would I change it, if I want to enable multiple Files to be uploaded?