Fix lightbox downloads for embeded images and a warning

Summary:
I refactored this recently and accidentally dropped the download URI.

Also fix a warning with, e.g., files named `README`.

Test Plan: Clicked a thumb, clicked "Download", got a file.

Reviewers: chad, btrahan, dctrwatson

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7341

src/applications/differential/view/DifferentialChangesetDetailView.php

@@ -47,7 +47,7 @@ final class DifferentialChangesetDetailView extends AphrontView {
 
   public function getFileIcon($filename) {
     $path_info = pathinfo($filename);
-    $extension = $path_info['extension'];
+    $extension = idx($path_info, 'extension');
     switch ($extension) {
       case 'psd':
       case 'ai':

src/applications/files/remarkup/PhabricatorRemarkupRuleEmbedFile.php

@@ -103,6 +103,7 @@ final class PhabricatorRemarkupRuleEmbedFile
         'meta'        => array(
           'phid' => $file->getPHID(),
           'uri' => $file->getBestURI(),
+          'dUri' => $file->getDownloadURI(),
           'viewable' => true,
         ),
       ),