This is because the blackboard_six qformat plugin needs the realfilename of the imported file and the qformat_default class in mod/lesson/import.php is missing the realfilename variable.

So what would be needed is to add var $realfilename = ''; in that class, create a method similar to public function setRealfilename($realfilename) { (see in question/format.php line 177) and use this method in mod/lesson/import.php in a similar way to what is done in question/import.php to populate this field.