config.xml

 <events>

                    <customer_address_save_after> <!-- The name of the Event -->
                    <observers>
                            <Ameex_Buy> <!-- Your module name -->
                                    <type>singleton</type>
                                    <class>Ameex_Buy_Model_Observer</class> <!-- The class name, that holds your callback function -->
                                    <method>handle_file_upload</method>
                            </Ameex_Buy>
                    </observers>
            </customer_address_save_after>
                </events>