How does koa-body get the information uploaded to the front-end page file?

Let"s briefly describe the project first

spa application of version 6.x of view layer angualr.js
angular calls the api API of node directly, and node invokes the service to add, delete, modify and query
node version 8.11.x mainly uses koa2 framework and koa-body (^ 4.0.4), koa-router (^ 7.1.1) as request resolution

requirements are as follows

now there is a file upload requirement, but node is only used for forwarding and does not actually save the file, but calls the service api to operate

// angular 
let formData:FormData = new FormData();
    formData.append("file",this.fileData);  // this.fileinput file 
    formData.append("name","");
    this.http.upload("assets/upload/add",formData, (data) => {
      console.log(data)
    }, (error) => {

    });
// node
const koaBody = require("koa-body");    
app.use(koaBody(
    multipart: true,
    formidable: {
      maxFileSize: 200 * 1024 * 1024 // 2M
    }
));

router.post("/upload/add", async (ctx, next) => {
  console.log(ctx.request.files);  // undefined
  console.log(ctx.request.body);   // {}
  ctx.body = JSON.stringify(ctx.request.files); // 
});

foreground page setup request header "Content-Type":" multipart/form-data;" sends data normally, and everything is normal.
now I want to upload the file to the server by calling the server api if I don"t store any files on the node side, but ctx.request.files doesn"t have any data. If there are other ways, you can also share