How to upload outlook email attachments on my cloud server using Outlook Js Add-in

I am developing outlook javascript add-in using vs2017. I have created a sample application to find attachments from outlook mail item. Here, While getting attachments from Exchange Server, It returns 200 OK.

I have my own cloud application looks like google drive. I want to upload outlook mail attachments on my cloud server using POST API call. API call was running successfully. But I am not able to get file content from the exchange server.

I have added some sample code over here.

• Creating a service request

          /// <reference path="../App.js" />
      var xhr;
      var serviceRequest;
  
      (function () {
          "use strict";
  
          // The Office initialize function must be run each time a new page is loaded
          Office.initialize = function (reason) {
              $(document).ready(function () {
                  app.initialize();
  
                  initApp();
              });
          };
  
          function initApp() {
              $("#footer").hide();
  
              if (Office.context.mailbox.item.attachments == undefined) {
                  var testButton = document.getElementById("testButton");
                  testButton.onclick = "";
                  showToast("Not supported", "Attachments are not supported by your Exchange server.");
              } else if (Office.context.mailbox.item.attachments.length == 0) {
                  var testButton = document.getElementById("testButton");
                  testButton.onclick = "";
                  showToast("No attachments", "There are no attachments on this item.");
              } else {
  
                  // Initalize a context object for the app.
                  //   Set the fields that are used on the request
                  //   object to default values.
                  serviceRequest = new Object();
                  serviceRequest.attachmentToken = "";
                  serviceRequest.ewsUrl = Office.context.mailbox.ewsUrl;
                  serviceRequest.attachments = new Array();
              }
          };
  
      })();
  
      function testAttachments() {
          Office.context.mailbox.getCallbackTokenAsync(attachmentTokenCallback);
      };
  
      function attachmentTokenCallback(asyncResult, userContext) {
          if (asyncResult.status == "succeeded") {
              serviceRequest.attachmentToken = asyncResult.value;
              makeServiceRequest();
          }
          else {
              showToast("Error", "Could not get callback token: " + asyncResult.error.message);
          }
      }
  
      function makeServiceRequest() {
          var attachment;
          xhr = new XMLHttpRequest();
          // Update the URL to point to your service location.
          xhr.open("POST", "https://localhost:8080/GetOutlookAttachments/AttachmentExampleService/api/AttachmentService", true);
  
          xhr.setRequestHeader("Content-Type", "application/json; charset=utf-8");
          xhr.onreadystatechange = requestReadyStateChange;
  
          // Translate the attachment details into a form easily understood by WCF.
          for (i = 0; i < Office.context.mailbox.item.attachments.length; i++) {
              attachment = Office.context.mailbox.item.attachments[i];
              attachment = attachment._data$p$0 || attachment.$0_0;
  
              if (attachment !== undefined) {
                  serviceRequest.attachments[i] = JSON.parse(JSON.stringify(attachment));
              }
          }
  
          // Send the request. The response is handled in the 
          // requestReadyStateChange function.
          xhr.send(JSON.stringify(serviceRequest));
      };
  
  
      // Handles the response from the JSON web service.
      function requestReadyStateChange() {
          if (xhr.readyState == 4) {
              if (xhr.status == 200) {
                  var response = JSON.parse(xhr.responseText);
                  if (!response.isError) {
                      // The response indicates that the server recognized
                      // the client identity and processed the request.
                      // Show the response.
                      var names = "<h2>Attachments processed: " + response.attachmentsProcessed + "</h2>";
                      document.getElementById("names").innerHTML = names;
                  } else {
                      showToast("Runtime error", response.message);
                  }
              } else {
                  if (xhr.status == 404) {
                      showToast("Service not found", "The app server could not be found.");
                  } else {
                      showToast("Unknown error", "There was an unexpected error: " + xhr.status + " -- " + xhr.statusText);
                  }
              }
          }
      };
  
      // Shows the service response.
      function showResponse(response) {
          showToast("Service Response", "Attachments processed: " + response.attachmentsProcessed);
      }
  
      // Displays a message for 10 seconds.
      function showToast(title, message) {
  
          var notice = document.getElementById("notice");
          var output = document.getElementById('output');
  
          notice.innerHTML = title;
          output.innerHTML = message;
  
          $("#footer").show("slow");
  
          window.setTimeout(function () { $("#footer").hide("slow") }, 10000);
      };
  

Please help me to get attachments from outlook mail and upload on my cloud server.

attachment._data$p$0 gives to attachment metadata. Get id from there and use getAttachmentContentAsync API to get the attachment content Documentation