Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 42407)
+++ src/wp-includes/functions.php	(working copy)
@@ -2555,6 +2555,7 @@
 			'vtt'                          => 'text/vtt',
 			'dfxp'                         => 'application/ttaf+xml',
 			// Audio formats.
+			'aac'                          => 'audio/aac',
 			'mp3|m4a|m4b'                  => 'audio/mpeg',
 			'ra|ram'                       => 'audio/x-realaudio',
 			'wav'                          => 'audio/wav',