Update file viewer sort order

We sort alphabetically now (with directories first) because we're
using the storage API, which returns directories and files separately.

In other news, hooray for more storage API coverage!

apps/files/tests/test_helpers.py

@@ -157,7 +157,7 @@ class TestFileHelper(amo.tests.TestCase):
         cache.clear()
         files = self.viewer.get_files().keys()
         rt = files.index(u'chrome')
-        eq_(files[rt:rt + 3], [u'chrome', u'chrome/foo', u'chrome.manifest'])
+        eq_(files[rt:rt + 3], [u'chrome', u'chrome/foo', u'dictionaries'])
 
     @patch.object(settings, 'FILE_VIEWER_SIZE_LIMIT', 5)
     def test_file_size(self):