Index: wp-includes/feed.php
===================================================================
--- wp-includes/feed.php	(revision 11326)
+++ wp-includes/feed.php	(working copy)
@@ -523,10 +523,10 @@
 		$type = get_default_feed();
 
 	$types = array(
-		'rss'  => 'application/rss+xml',
-		'rss2' => 'application/rss+xml',
+		'rss'  => 'text/xml',
+		'rss2' => 'text/xml',
 		'atom' => 'application/atom+xml',
-		'rdf'  => 'application/rdf+xml',
+		'rdf'  => 'text/xml',
 	);
 
 	$content_type = ( !empty($types[$type]) ) ? $types[$type] : 'application/octet-stream';