Add Content-Length header to Aphront file responses.

Summary:
Provide a Content-Length header so that browsers can estimate time
remaining for file downloads.

Test Plan: Tested on our local phabricator install.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6107

src/aphront/response/AphrontFileResponse.php

@@ -43,6 +43,7 @@ final class AphrontFileResponse extends AphrontResponse {
   public function getHeaders() {
     $headers = array(
       array('Content-Type', $this->getMimeType()),
+      array('Content-Length', strlen($this->content)),
     );
 
     if (strlen($this->getDownload())) {