[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.

This PR is for develop-ascraeus and above and contains tests. The develop-olympus one can be found here: https://github.com/phpbb/phpbb/pull/2237

Ticket: http://tracker.phpbb.com/browse/PHPBB3-10851