Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 12692)
+++ wp-includes/functions.php	(working copy)
@@ -2284,7 +2284,7 @@
 		'rtx' => 'text/richtext',
 		'css' => 'text/css',
 		'htm|html' => 'text/html',
-		'mp3|m4a' => 'audio/mpeg',
+		'mp3|m4a|m4b' => 'audio/mpeg',
 		'mp4|m4v' => 'video/mp4',
 		'ra|ram' => 'audio/x-realaudio',
 		'wav' => 'audio/wav',