wp-attachment-gallery.php

<?php 
	$args = array(
		'post_type' => 'attachment',
		'numberposts' => -1,
		'post_status' => null,
		'post_parent' => $post->ID
	);

	$attachments = get_posts( $args );

	if ( $attachments ) {
		echo '<ul id="gallery" class="thumb-grid">';
			foreach ( $attachments as $attachment ) {
			$large_image = wp_get_attachment_image_src($attachment->ID,'large',false);
			echo '<li><a href="' . $large_image[0] . '">'. wp_get_attachment_image( $attachment->ID, 'thumbnail' ) .'</a></li>';
		}
		echo '</ul>';
	}
?>