cache_timeout should be always checked not Null
diff --git a/flask/helpers.py b/flask/helpers.py
index 72a961a827..18502a53f0 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -533,7 +533,7 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
     rv.cache_control.public = True
     if cache_timeout is None:
         cache_timeout = current_app.get_send_file_max_age(filename)
-    if cache_timeout:
+    if cache_timeout is not None:
         rv.cache_control.max_age = cache_timeout
         rv.expires = int(time() + cache_timeout)
 

I think send_file should always check that cache_timeout is not None to allow for a (I hope legal) value of 0 for such parameter.
