[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
[ticket/10851] Request index instead of logging in and out in tests

PHPBB3-10851
Merge remote-tracking branch 'Marc/ticket/10851' into develop-olympus

* Marc/ticket/10851:
  [ticket/10851] Set disallowed content to empty array if checking is disabled
Merge remote-tracking branch 'Marc/ticket/10851-ascraeus' into develop-ascraeus

* Marc/ticket/10851-ascraeus:
  [ticket/10851] Request index instead of logging in and out in tests
  [ticket/10851] Add unit and functional tests for checking attachments
  [ticket/10851] Set disallowed content to empty array if checking is disabled
Merge branch 'develop-olympus' into develop-ascraeus

* develop-olympus: