Provide the attachment_id of a media item in the XML-RPC call wp.getMediaItem.

Props skithund
Fixes #18683


git-svn-id: http://svn.automattic.com/wordpress/trunk@19885 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-xmlrpc-server.php

@@ -2260,6 +2260,7 @@ class wp_xmlrpc_server extends IXR_Server {
 			'caption'				=> $attachment->post_excerpt,
 			'description'			=> $attachment->post_content,
 			'metadata'				=> wp_get_attachment_metadata($attachment->ID),
+			'attachment_id'			=> (string) $attachment->ID
 		);
 
 		return $attachment_struct;