Backport PR #1143: Exempt javascript files from files check

Closes #1040.

External JS files are loaded with `application/javascript` mimetype.

cc: @Carreau @michaelsbradleyjr

notebook/static/tree/js/notebooklist.js

@@ -555,6 +555,7 @@ define([
         var uri_prefix = NotebookList.uri_prefixes[model.type];
         if (model.type === 'file' &&
             model.mimetype && model.mimetype.substr(0,5) !== 'text/'
+            && !model.mimetype.endsWith('javascript')
         ) {
             // send text/unidentified files to editor, others go to raw viewer
             uri_prefix = 'files';