[ticket/14774] Support partial downloads of attachments
phpBB already had limited support for partial downloads, but only for
resuming downloads, disregarding any range ending before EOF.
WebKit on iOS and OS X uses partial downloads when fetching media
files. Previously, only MP3 attachments could play directly in the
browser, reported as a live stream, but with this change, all
supported media formats should render as expected.
Tested using cURL by verifying that partial downloads give exactly the
same results compared to Apache.
