Updated to to use a specific custom image size, with no fall back - better to have no image instead

Wordpress RSS 2.0 image enclosure

function feedFilter($query) {
	if ($query->is_feed) {
		add_filter('rss2_item', 'feedContentFilter');
		}
	return $query;
}
add_filter('pre_get_posts','feedFilter');
 
function feedContentFilter($item) {
	
	global $post;
	
	
	$attachment_id = get_post_thumbnail_id($post->ID);
	$mime = get_post_mime_type($attachment->ID);
	$image_info = wp_get_attachment_image_src($attachment_id, 'custom-image-size');
	
	if ($image_info) {
		echo '<enclosure url="'.$image_info[0].'" length="" type="'.$mime.'"/>';
	}
	
	return $item;
}