Index: file.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/file.inc,v
retrieving revision 1.121.2.11
diff -u -p -r1.121.2.11 file.inc
--- file.inc	1 Mar 2010 09:51:16 -0000	1.121.2.11
+++ file.inc	2 Jun 2010 18:33:32 -0000
@@ -811,6 +811,14 @@ function file_transfer($source, $headers
   if (ob_get_level()) {
     ob_end_clean();
   }
+  
+  // IE cannot download private files because it cannot store files downloaded
+  // over https in the browser cache. The problem can be solved by sending
+  // custom headers to IE. See http://support.microsoft.com/kb/323308/en-us
+  if (isset($_SERVER['HTTPS']) && ($_SERVER['HTTPS'] == 'on')) {
+    drupal_set_header('Cache-Control: private');
+    drupal_set_header('Pragma: private');
+  }
 
   foreach ($headers as $header) {
     // To prevent HTTP header injection, we delete new lines that are