Updates an existing file field with new settings.
function updateFileField($name, $type_name, $instance_settings = array(), $widget_settings = array()) {
$instance = field_info_instance('node', $name, $type_name);
$instance['settings'] = array_merge($instance['settings'], $instance_settings);
$instance['widget']['settings'] = array_merge($instance['widget']['settings'], $widget_settings);
field_update_instance($instance);
}