Incorrect Control-cache in /system/helpers/download_helper.php

If both "post-check" and "pre-check" are specified and set to 0, both are entirely ignored.

See reference :
http://blogs.msdn.com/b/ieinternals/archive/2009/07/20/using-post_2d00_check-and-pre_2d00_check-cache-directives.aspx