Angular FormData SyntaxError:JSON中位于JSON位置0处的意外标记U

我的前台服务器是Angular6

后台服务器是express

当我尝试将formdata发布到backserver时

文件上载成功

但是我得到了这个错误

SyntaxError: Unexpected token U in JSON at position 0
    at JSON.parse (<anonymous>)
    at XMLHttpRequest.onLoad (webpack-internal:///./node_modules/@angular/common/esm5/http.js:2289:51)
    at ZoneDelegate.invokeTask (webpack-internal:///./node_modules/zone.js/dist/zone.js:421:31)
    at Object.onInvokeTask (webpack-internal:///./node_modules/@angular/core/esm5/core.js:4967:33)
    at ZoneDelegate.invokeTask (webpack-internal:///./node_modules/zone.js/dist/zone.js:420:36)
    at Zone.runTask (webpack-internal:///./node_modules/zone.js/dist/zone.js:188:47)
    at ZoneTask.invokeTask [as invoke] (webpack-internal:///./node_modules/zone.js/dist/zone.js:496:34)
    at invokeTask (webpack-internal:///./node_modules/zone.js/dist/zone.js:1540:14)
    at XMLHttpRequest.globalZoneAwareCallback (webpack-internal:///./node_modules/zone.js/dist/zone.js:1566:17)

http方法

  User_postFileData_withF(url: string, body) {
    const headers = new HttpHeaders({'Authorization': sessionStorage.getItem("userToken") });
    return this.http.post(this.serverip + url, body, { headers: headers });
  }

我的html

  <input type="file" accept="image/*" (change)="changeFile($event)" 

  <button type="button" class="btn btn-primary" (click)="uploadFile()">上傳</button>

多谢各位

  <input type="file" accept="image/*" (change)="changeFile($event)" 

  <button type="button" class="btn btn-primary" (click)="uploadFile()">上傳</button>

  uploadData = new FormData();

  changeFile(event) {
    var image = event.target.files[0];

    var pattern = /image-*/;
    var reader = new FileReader();

    this.uploadData.append('name', image);
  }