customizing custom fields

I solved this myself by poking around in wp-admin/includes/media.php

function my_image_attachment_fields_to_edit($form_fields, $post) {
	$pte = pte_media_row_actions("", $post, "");
	$pte = $pte['pte'];
	$form_fields["custom_sizes"] = array(
			"label" => __("Sizes"),
			"input" => "html",
			"html" => $pte
	);
	return $form_fields;
}

add_filter("attachment_fields_to_edit", "my_image_attachment_fields_to_edit", null, 2);