无法在angular 6中通过formdata上传文件

<input type="file" (change)="onFileSelected($event.target.files)">
<button (click)="onUpload()">上传</button>

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Component({
  selector: 'app-upload',
  templateUrl: './upload.component.html',
  styleUrls: ['./upload.component.css']
})
export class UploadComponent {
  selectedFile: File = null;

  constructor(private http: HttpClient) { }

  onFileSelected(files: FileList) {
    this.selectedFile = files.item(0);
  }

  onUpload() {
    if (this.selectedFile) {
      const formData = new FormData();
      formData.append('file', this.selectedFile, this.selectedFile.name);
      this.http.post('http://your-upload-url', formData)
        .subscribe(res => {
          console.log(res);
          // 处理上传成功的响应
        }, error => {
          console.log(error);
          // 处理上传失败的响应
        });
    }
  }
}