angular2 formdata是空的

Question

service.ts

uploadFile(event) {
  //let userForm = document.forms.namedItem('userSetting');
  let formData = new FormData();
  formData.append('file', document.getElementById('avatar')['files'][0]);
  console.log(event, '=============>event');
  console.log(document.getElementById('avatar')['files'][0], '=============>document.getElementById');
  // form.append('img', document.getElementsByName('avatar')[0]['files'][0]);
  console.log(formData, '================>uploadfile');
}

component.html

<div class="form-group">
    <label class="col-sm-2 control-label">头像：</label>
    <div class="col-sm-10">
        <input type="file" name= 'avatar' id="avatar" class="form-control" placeholder="上传头像" (change)="uploadFile($event)">
    </div>
</div>[why formdata is empty?][1]

表单数据看起来不行，我的代码有什么问题吗？