[WordPress] Add current ancestor class to nav menu when viewing a singular attachment page, e.g. via the image.php template file.

current-attachment-nav-class.php

<?php
/**
 * Add current item classes for attachment views
 */
function _s_current_attachment_nav_class( $classes, $menu_item ) {
	
	global $wp_query, $post;

	if( true == $wp_query->is_singular && 'attachment' == $post->post_type && in_array( $menu_item->object_id, $post->ancestors ) ) {
		$classes[] = 'current-menu-ancestor';
	}

	return $classes;
}
add_filter( 'nav_menu_css_class', '_s_current_attachment_nav_class', 10, 2 );