Index: src/wp-includes/feed.php
===================================================================
--- src/wp-includes/feed.php	(revision 40036)
+++ src/wp-includes/feed.php	(working copy)
@@ -644,7 +644,7 @@
 		'rdf'      => 'application/rdf+xml'
 	);
 
-	$content_type = ( !empty($types[$type]) ) ? $types[$type] : 'application/octet-stream';
+	$content_type = ( !empty($types[$type]) ) ? $types[$type] : 'text/xml';
 
 	/**
 	 * Filters the content type for a specific feed type.