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add_filter('tribe_ical_feed_item','tribe_add_attachment_to_ical', 10, 2);

function tribe_add_attachment_to_ical($item, $event){

	$args = array(
		'post_type' => 'attachment',
		'numberposts' => -1,
		'post_status' => null,
		'post_parent' => $event->ID
	);
	
	$size = 'full';
	
	$attachments = get_posts($args);
	if ($attachments) {
		foreach ($attachments as $attachment) {
			$item[] = "ATTACH:".wp_get_attachment_image_src($attachment->ID, $size);		
			
		}
	}

	return $item;
}