Index: attachment.py
===================================================================
--- attachment.py       (revision 1504)
+++ attachment.py       (working copy)
@@ -197,6 +197,7 @@
         req.send_header('Content-Type', mime_type)
         req.send_header('Content-Length', str(length))
         req.send_header('Last-Modified', util.http_date(last_modified))
+        req.send_header('Cache-Control', 'no-store')
         req.end_headers()
 
         while data: