WordPress at a glance
action is not described

attachment_updated action-hook . WP 4.4.0

Fires once an existing attachment has been updated.

Usage

add_action( 'attachment_updated', 'action_function_name_3976', 10, 3 );
function action_function_name_3976( $post_ID, $post_after, $post_before ){
	// action...
}
$post_ID(int)
Post ID.
$post_after(WP_Post)
Post object following the update.
$post_before(WP_Post)
Post object before the update.

Where the hook is called

wp_insert_post()
attachment_updated
wp-includes/post.php 3684
do_action( 'attachment_updated', $post_ID, $post_after, $post_before );

Where the hook is used (in WP core)

wp-includes/default-filters.php 377
add_action( 'attachment_updated', 'wp_check_for_changed_slugs', 12, 3 );
wp-includes/default-filters.php 381
add_action( 'attachment_updated', 'wp_check_for_changed_dates', 12, 3 );