The best way of doing this might be to attach a custom field to each post to keep the increment {n}.
Something to the effect of:
$attach_inc = 1;
if ( is_single() ) {
if ( $attach_inc = get_post_meta( $post_id, 'attachment_inc', true ) ) {
$attach_inc++;
update_post_meta( $post_id, 'attachment_inc', $attach_inc );
} else {
add_post_meta( $post_id, 'attachment_inc', $attach_inc );
}
} else {
if ( $attach_inc = get_option( 'attachment_inc' ) ) {
$attach_inc++;
set_option( 'attachment_inc', $attach_inc );
} else {
add_option( 'attachment_inc', $attach_inc );
}
}
// DO YOUR ADD ATTACHMENT STUFF HERE