mail.php

<?php 
//The form has been submitted, prep a nice thank you message
$output = '<h1>!</h1>';

//Set the form flag to no display (cheap way!)
$flags = 'style="display:none;"';

//Deal with the email
$to = 'ganadores@XXX.com';
$subject = 'Subasta slip';

/* TEST: A partir de aquí sobrescrito variables para probar de manera más comoda */
//$message = strip_tags($_POST['message']);
$message = strip_tags('Aquí iría el POST'); //Borra
//$attachment = chunk_split(base64_encode(file_get_contents($_FILES['file']['tmp_name'])));
$attachment = 'Aquí el atachment'; //Borra
//$filename = $_FILES['file']['name'];
$filename = 'Aquí el fileName'; //Borra
/* Creo variable ID, ya que no aparece en el código */
$id = 'TEST';
/* FIN TEST */

$boundary =md5(date('r', time())); 

$headers = "From: cliente@XXX.com\r\nReply-To: cliente@XXX.com";
$headers .= "\r\nMIME-Version: 1.0\r\nContent-Type: multipart/mixed; boundary=\"_1_$boundary\"";

$message="This is a multi-part message in MIME format.

--_1_$boundary
Content-Type: multipart/alternative; boundary=\"_2_$boundary\"

--_2_$boundary
Content-Type: text/html; charset=\"iso-8859-1\"
Content-Transfer-Encoding: 7bit

<h2>ID del producto</h2><p> $id </p> $message

--_2_$boundary--
--_1_$boundary
Content-Type: application/octet-stream; name=\"$filename\" 
Content-Transfer-Encoding: base64 
Content-Disposition: attachment 

$attachment
--_1_$boundary--";

mail($to, $subject, $message, $headers);