diff --git a/IPython/html/static/edit/js/editor.js b/IPython/html/static/edit/js/editor.js
index c1f607b..481507c 100644
--- a/IPython/html/static/edit/js/editor.js
+++ b/IPython/html/static/edit/js/editor.js
@@ -100,7 +100,9 @@ function($,
 
         // Find and load the highlighting mode,
         // first by mime-type, then by file extension
+
         var modeinfo;
+        // mimetype is unset on file rename
         if (model.mimetype) {
             modeinfo = CodeMirror.findModeByMIME(model.mimetype);
         }