Fix place where we were passing an Array[string] to CodeMirror.findModeByExtension instead of passing a string. (I guess older versions of CodeMirror allowed this?)

Author: bzinberg

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};