Register a custom extended property in this item class so they can be accessed just like any other attribute

def register(cls, attr_name, attr_cls):
        """
        Register a custom extended property in this item class so they can be accessed just like any other attribute
        """
        if not cls.INSERT_AFTER_FIELD:
            raise ValueError('Class %s is missing INSERT_AFTER_FIELD value' % cls)
        try:
            cls.get_field_by_fieldname(attr_name)
        except InvalidField:
            pass
        else:
            raise ValueError("'%s' is already registered" % attr_name)
        if not issubclass(attr_cls, ExtendedProperty):
            raise ValueError("%r must be a subclass of ExtendedProperty" % attr_cls)
        # Check if class attributes are properly defined
        attr_cls.validate_cls()
        # ExtendedProperty is not a real field, but a placeholder in the fields list. See
        #   https://msdn.microsoft.com/en-us/library/office/aa580790(v=exchg.150).aspx
        #
        # Find the correct index for the new extended property, and insert.
        field = ExtendedPropertyField(attr_name, value_cls=attr_cls)
        cls.add_field(field, insert_after=cls.INSERT_AFTER_FIELD)