Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 18323)
+++ wp-includes/formatting.php	(working copy)
@@ -2907,7 +2907,7 @@
  * @return string Sanitized mime type
  */
 function sanitize_mime_type( $mime_type ) {
-	$sani_mime_type = preg_replace( '/[^-*.a-zA-Z0-9\/]/', '', $mime_type );
+	$sani_mime_type = preg_replace( '/[^-+*.a-zA-Z0-9\/]/', '', $mime_type );
 	return apply_filters( 'sanitize_mime_type', $sani_mime_type, $mime_type );
 }