XML-RPC: Return error message if attachment ID is incorrect.

Throw an error for incorrect attachment IDs when requesting a media object to ensure return signature is correct and doesn't include incorrectly typed values within the object.

Props zieladam, peterwilsoncc, whyisjake.
Merges [49376] to trunk.
See #49905.
Built from https://develop.svn.wordpress.org/trunk@49385


git-svn-id: http://core.svn.wordpress.org/trunk@49144 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -4311,7 +4311,7 @@ class wp_xmlrpc_server extends IXR_Server {
 		do_action( 'xmlrpc_call', 'wp.getMediaItem' );
 
 		$attachment = get_post( $attachment_id );
-		if ( ! $attachment ) {
+		if ( ! $attachment || 'attachment' !== $attachment->post_type ) {
 			return new IXR_Error( 404, __( 'Invalid attachment ID.' ) );
 		}