Index: includes/file.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/file.inc,v
retrieving revision 1.121.2.10
diff -u -p -r1.121.2.10 file.inc
--- includes/file.inc	1 Feb 2010 16:14:58 -0000	1.121.2.10
+++ includes/file.inc	3 Feb 2010 00:44:27 -0000
@@ -803,6 +803,15 @@ function file_transfer($source, $headers
     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
+  global $base_url;
+  if (preg_match('|^https://|', $base_url)) {
+    drupal_set_header('Cache-Control: private');
+    drupal_set_header('Pragma: private');
+  }
+
   foreach ($headers as $header) {
     // To prevent HTTP header injection, we delete new lines that are
     // not followed by a space or a tab.