What is "$ _FILES Corruption Attack"

I came across the following piece of code for checking 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 "$ _FILES Corruption Attack"?
(How) does the mentioned Code prevent it?
How can I change it if I want to include multiple downloads?