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<?
add_filter('wp_get_attachment_image_attributes', 'change_attachement_image_attributes', 20, 2);
function change_attachement_image_attributes($attr, $attachment) {
    global $post;
    if ($post->post_type == 'product') {
        $title               =  $post->post_title;
        static $num          =  0;
        $num++;
        $attr['alt']         =  sprintf("Фото %d - %s.",$num,$title);
    }
    return $attr;
}