[ticket/10851] Set disallowed content to empty array if checking is disabled

The disallowed content defaults to a standard set of mimetype triggers by
default. If one doesn't want to check the attachments mimetype triggers then
we need to set the disallowed content to an empty array.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-10851

Does not contain tests as test cases for fileupload do not exist in develop-olympus.
Tests are added for develop-ascraeus and develop in a seperate PR for those branches.