PHP - Force file to download - not working

I'm trying to force the download of a file instead of displaying it in a browser and the below code allows me to download the file, but seems not to get the full file.  I can not display any of the jpgs that I download.  Any ideas on what I have wrong?  Keep in mind images may be just one type of a download, as there could be .txt or .pdf etc...

<?php
$file="c:\upload_test\\robot-100_1.jpg"; //file location 
header("Content-Description: File Transfer"); 
header("Content-Type: application/octet-stream");
header('Content-Disposition: attachment; filename="'.basename($file).'"'); 
header('Content-Length: ' . filesize($file));
header('Connection: close');
readfile($file);

?>

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