const req = new HttpRequest('POST', '/upload/file', file, {
  reportProgress: true,
});

uploadPayrolls(files: File[], payrollDocId: string) {
    const formData = new FormData();
    files.forEach(file => {
      formData.append('files', file);
    });
    return this.http.post(
      `${this.apiUrl}/payrolls/upload`,
      formData,
      {
        params: { payrollDocId },
        reportProgress: true,
        observe: 'events'
      }
    ).pipe(
      distinctUntilChanged(),
      map(this.processProgress)
    );
  }

  processProgress(event) {
    switch (event.type) {
      case HttpEventType.UploadProgress:
        const progress = Math.round(100 * event.loaded / event.total);
        return { status: 'progress', progress };
      case HttpEventType.Response:
        return event.body;
      default:
        return `Unhandled event: ${event.type}`
    }
  }