[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

PHPBB3-10820
[ticket/10820] proper usage of global and local variable browser

PHPBB3-10820
[ticket/10820] fix IE check function

Add phpbb_ prefix to function name and return boolean value.

PHPBB3-10820
[ticket/10820] add function docblock

PHPBB3-10820
[ticket/10820] remove unnecessary condition checks

PHPBB3-10820