post_gallery filter not passing arguments of gallery shortcode

I have a plugin like so:

<?php
/* Author info */

add_filter('post_gallery', 'kg_gallery_shortcode');

function kg_gallery_shortcode($attr) {
	global $post;

	extract(shortcode_atts(array(
		'id' => $post->ID,
		'ex' => ''
	), $attr));

/* ... */
}
?>

Well the plugin works fine doing everything I need to do with the default [gallery] tag, but it seems like it completely ignores any attributes, e.g. [gallery ex="2"] or even [gallery id="1"].

I tried changing the filter addition to

add_filter('post_gallery', 'kg_gallery_shortcode', 1, 1);

But that changed nothing. Why isn’t anything coming through on the argument array?

Thanks.