title: Send a file from facebook messenger (api.ai)
body: <p>I'm creating a bot for facebook messenger with api.ai,in the doc says that i must to write the following code on the 'custom payload' option:</p>
  
  <pre><code>"message":{
      "attachment":{
        "type":"file",
        "payload":{
          "url":"https://petersapparel.com/bin/receipt.pdf"
        }
      }
  }
  </code></pre>
  
  <p>but it doesnt work, someone know if I missed something? The url that I use works correctly.</p>