Add an attribute to the MediaContent element.

def _add_attribute(self, name, value, allowed_values=None):
        """Add an attribute to the MediaContent element."""
        if value and value != 'none':

            if isinstance(value, (int, bool)):
                value = str(value)

            if allowed_values and value not in allowed_values:
                raise TypeError(
                    "Attribute '" + name + "' must be one of " + str(
                        allowed_values) + " but is '" + str(value) + "'")

            self.element_attrs[name] = value