Ok, that makes sense. I saw it on demo.moodle.net.
Hi Karoly,
you are actually looking for mod_assignment. Since Moodle 2.0 we have started slow and long process of correct referencing of Moodle components so from now on, every Moodle plugin should be called as type_name. For activity modules, there is a nasty exception yet that they do not need to have mod_ prefix. Therefore, the name of the file is assignment.php but the component is mod_assignment.