Add filetype restriction to assign module

When adding an assign module to a course, there isn't any option to restrict a student to submit a particular filetype(s) only. It will be useful to have this feature in the core assign moodle module.

I added this functionality in Moodle 2.3, in my early days of Moodle Development when I was not aware of how to respect Core Moodle code.

I now want to offer this improvement as part of official core moodle build.