[ticket/10820] Image downloader recognize new version of ie

When a user download image attachement using ie8, the file is displayed.
However, when he uses ie version greater than 8, the image is download.
A changes are made to phpbb/download/file.php to solve the problem.
We check now if the ie version is greater or equal to 8 and not only equal
to 8
http://tracker.phpbb.com/browse/PHPBB3-10820
PHPBB3-10820