php readfile does not download

I am testing a small bit of code to download a file but nothing downloads. I have looked at countless examples (I know this has been asked many times) and from reading these examples this should work. The file definitely exists but no download.

What am I doing wrong?

This is the file (called download.php) in its entirety.

<?php

$file = $_SERVER["DOCUMENT_ROOT"] . "/uploads/test.png" ;
if (file_exists($file)) {
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename="'.basename($file).'"');
    header('Expires: 0');
    header('Cache-Control: must-revalidate');
    header('Pragma: public');
    header('Content-Length: ' . filesize($file));
    readfile($file);
    exit;
}

It is called with a simple js function:

function download() {

    $.post("download.php", 
        function(data, status){
          ... some stuff will go in here
        }
    );
}