wp_get_recent_posts and post_type attachment

Hi there,

I need some help.

Im writing simple plugin displaying last uploaded photo. I try use this:

$args = array(
                'numberposts' => 1,
                'orderby' => 'post_date',
                'order' => 'DESC',
                'post_type' => 'attachment',
            );

            $post = wp_get_recent_posts($args);

and $post == null.
I have no idea why except wordpress does not like me.