explode('|', '') and explode('|', NULL) both return array(0 => '') which can
cause filespec::check_content() to reject everything starting with a '<'
character in case $config['mime_triggers'] is an empty string or not set.
fileupload::set_disallowed_content() now filters out empty strings by calling
array_diff() on the passed array, so setting $config['mime_triggers'] to an
empty string will turn off mime checking completely.
On the other side we want to fail safe if $config['mime_triggers'] is not set
at all. To do this, the array fileupload::$disallowed_content now contains some
default strings to be filtered out.
PHPBB3-9764
ac26bb458f