title: Unable to send pdf file from bot to user in ms teams
body: <p>NodeJS BotBuilder SDK version: 3.15.0
  My code: </p>
  
  <pre><code>var pdf = {
    name: '&lt;file_name&gt;.pdf',
    contentType: 'application/pdf',
    contentUrl: '&lt;https url to public pdf file&gt;'
  };
  var reply = new builder.Message(session).addAttachment(pdf);
  session.send(reply);
  </code></pre>
  
  <p>This code is the same in few online examples. The issue I have is that I always get error: </p>
  
  <pre><code>Error: POST to 'https://smba.trafficmanager.net/emea/v3/conversations/a%3A1TwHmhoGuZP2Mf9P0TTnjv8HkcaXzEHryv0sYCvDDUI-qrMitJtHRlAnIcedcDH_v3IfMBXtg_zo5MDVcS0-8hDCQ4sJzpJhrewBPK8uWJXYeShgmd-s7uh5o8kW4ebAP/activities/1543588440246' failed: [400] Bad Request
  </code></pre>
  
  <p>For image/png this code works fine. </p>
  
  <p>What I want to achieve is this: (image is taken from Bot Framework Emulator) 
  <a href="https://i.stack.imgur.com/socx2.png" rel="nofollow noreferrer">File from the web sent from bot to user</a></p>
  
  <p>The file is sent from bot without uploading it to users's one drive.
  This works also when I tested the feature in test section of <a href="https://dev.botframework.com/bots" rel="nofollow noreferrer">https://dev.botframework.com/bots</a>. It doesn't work only in ms teams. </p>