Index: file.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/file.inc,v
retrieving revision 1.121.2.12
diff -u -r1.121.2.12 file.inc
--- file.inc	11 May 2010 09:49:58 -0000	1.121.2.12
+++ file.inc	10 Jul 2010 10:19:23 -0000
@@ -826,6 +826,15 @@
   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');
+    drupal_set_header('Content-Disposition: attachment; filename="'.basename($source).'"');
+  }
 
   foreach ($headers as $header) {
     // To prevent HTTP header injection, we delete new lines that are