title: Facebook send API sending audio issue
body: <p>Can't send audio through facebook send api. It always returns with an HTTP 400 Bad Request. I use python and urllib for the http request.</p>
  
  <p>Json: </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>   'recipient':{
    'id' : userId
   },
   'message' : {
    'attachment' : {
     'type' : 'audio',
     'payload' : {
      'url' : url
     }
    }
   }
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>Request:</p>
  
  <pre><code>def send_msg(userId,data,accessKey):
      HEADERS = {
          'Content-Type':'application/json'
      }
      url = 'https://graph.facebook.com/v2.6/me/messages?access_token='+accessKey
      msg_json = {
         'message' : {
          'attachment' : {
              'type' : 'audio',
              'payload' : {
                  'url' : url
              }
          }
      }
  
  
      data = json.dumps(msg_json).encode('utf-8')
      req = urllib.request.Request(url,data,HEADERS)
      resp = urllib.request.urlopen(req)
  </code></pre>
  
  <p>The code works on image and file attachment but it doesn't work on audio.</p>