Not able to receive an attachment from Skype in BotFramework

On BotFramework (NodeJS), I was trying to replicate the demo available at https://learn.microsoft.com/en-us/bot-framework/nodejs/bot-builder-nodejs-send-receive-attachments . It actually works well.

Code in case ms article change:

// Create your bot with a function to receive messages from the user
var bot = new builder.UniversalBot(connector, function (session) {
    var msg = session.message;
    if (msg.attachments && msg.attachments.length > 0) {
     // Echo back attachment
     var attachment = msg.attachments[0];
        session.send({
            text: "You sent:",
            attachments: [
                {
                    contentType: attachment.contentType,
                    contentUrl: attachment.contentUrl,
                    name: attachment.name
                }
            ]
        });
    } else {
        // Echo back users text
        session.send("You said: %s", session.message.text);
    }
});

However, the problem I am facing is that when I do a call from Skype (Normal), I receive an error message:

2017-12-07T02:16:15.815Z Error: POST to 'https://smba.trafficmanager.net/apis/v3/conversations/<My Conversation>/activities' failed: [400] Bad Request
    at Request._callback (/app/node_modules/botbuilder/lib/bots/ChatConnector.js:545:46)
    at Request.self.callback (/app/node_modules/request/request.js:186:22)
    at emitTwo (events.js:126:13)
    at Request.emit (events.js:214:7)
    at Request.<anonymous> (/app/node_modules/request/request.js:1163:10)
    at emitOne (events.js:116:13)
    at Request.emit (events.js:211:7)
    at IncomingMessage.<anonymous> (/app/node_modules/request/request.js:1085:12)
    at Object.onceWrapper (events.js:313:30)
    at emitNone (events.js:111:20)

Any ideas?

[Update: It happens only when I create the attachment response. So I expect that's where I have an issue]

Actually the code on the MS Website is not up-to-date (in a way).

If I follow the code visible at : https://github.com/Microsoft/BotBuilder-Samples/tree/master/Node/core-ReceiveAttachment

By example, I can receive the attachment and save it somewhere on a public folder. Once completed, then I can send back the "public" URL as attachment and then it works.