The problem is that image is not a proper mime type. You can see a list of mime-types here – so based off the linked list we need to pass an array of what we need:
$supported_mimes = array(
'image/jpeg', 'image/gif', 'image/png',
'video/avi', 'video/x-flv', 'video/mp4', 'video/ogg', 'video/webm',
);
$args = array(
'numberposts' => -1, // Using -1 loads all posts
'orderby' => 'menu_order', // This ensures images are in the order set in the page media manager
'order' => 'ASC',
'post_mime_type' => $supported_mimes
'post_parent' => $post->ID, // Important part - ensures the associated images are loaded
'post_status' => null,
'post_type' => 'attachment'
);
I think the bigger problem is enabling WordPress to allow you to create a gallery with both images and videos but as far as this goes, this is just pulling media uploaded to that post.