diff --git a/notebook/static/base/js/utils.js b/notebook/static/base/js/utils.js
index 54a2bd0d4..1d8a9d010 100644
--- a/notebook/static/base/js/utils.js
+++ b/notebook/static/base/js/utils.js
@@ -754,7 +754,7 @@ define([
         // .mime
         var info = (mode && mode.mode && mode.mime && mode) ||
             CodeMirror.findModeByName(modename) ||
-            CodeMirror.findModeByExtension(modename.split(".").slice(-1)) ||
+            CodeMirror.findModeByExtension(modename.split(".").slice(-1)[0]) ||
             CodeMirror.findModeByMIME(modename) ||
             {mode: modename, mime: modename};