Index: includes/file.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/file.inc,v
retrieving revision 1.116
diff -u -p -r1.116 file.inc
--- includes/file.inc	28 Dec 2007 10:50:51 -0000	1.116
+++ includes/file.inc	31 Dec 2007 22:00:06 -0000
@@ -792,7 +792,9 @@ function file_set_status(&$file, $status
  * @param $headers An array of http headers to send along with file.
  */
 function file_transfer($source, $headers) {
-  ob_end_clean();
+  if (ob_get_length() > 0) {
+    ob_end_clean();
+  }
 
   foreach ($headers as $header) {
     // To prevent HTTP header injection, we delete new lines that are