[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.

PHPBB3-10851
Merge branch 'ticket/10851' into ticket/10851-ascraeus
[ticket/10851] Add unit and functional tests for checking attachments

These tests will make sure that fileuploads now work as expected regarding
the check for disallowed content.

PHPBB3-10851