Index: wp-includes/class-wp-xmlrpc-server.php
===================================================================
--- wp-includes/class-wp-xmlrpc-server.php	(revision 18690)
+++ wp-includes/class-wp-xmlrpc-server.php	(working copy)
@@ -1596,6 +1596,7 @@
 			'caption'				=> $attachment->post_excerpt,
 			'description'			=> $attachment->post_content,
 			'metadata'				=> wp_get_attachment_metadata($attachment->ID),
+			'attachment_id'			=> $attachment->ID
 		);
 
 		return $attachment_struct;