cache control header

This patch adds a cache control header to fetch.php. Without
this header media files can't be opened and viewed with third
party programs directly from the IE browser window. For eg.
Acrobat Reader will display an error message after clicking
on a link to a PDF file. Firefox will work without it.

darcs-hash:20050606154420-7ef76-2b46b0eb626488cec122a00cc32f86a28542b161.gz

lib/exe/fetch.php

@@ -73,6 +73,7 @@
   header("Content-Type: $MIME");
   header('Last-Modified: '.date('r',filemtime($FILE)));
   header('Content-Length: '.filesize($FILE));
+  header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
 
   //application mime type is downloadable
   if(substr($MIME,0,11) == 'application'){