asp.net-web-api – 如何从ASP.net 5 web api返回文件

{
  "version": {
    "major": 1,"minor": 1,"build": -1,"revision": -1,"majorRevision": -1,"minorRevision": -1
  },"content": {
    "headers": [
      {
        "key": "Content-Disposition","value": [
          "attachment; filename=test.pdf"
        ]
      },{
        "key": "Content-Type","value": [
          "application/pdf"
        ]
      }
    ]
  },"statusCode": 200,"reasonPhrase": "OK","headers": [],"requestMessage": null,"isSuccessStatusCode": true
}`

码：

public HttpResponseMessage Post([FromBody]DocumentViewModel vm)
    {
        try
        {
            if (ModelState.IsValid)
            {

                var Document = _repository.GetDocumentByGuid(vm.DocumentGuid,User.Identity.Name);
                var Params = Helper.ClientInputToRealValues(vm.Parameters,Document.DataFields);
                var file = Helper.GeneratePdf(Helper.InsertValues(Params,Document.Content));                 

                var result = new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = new ByteArrayContent(System.IO.File.ReadAllBytes(file))
                };
                result.Content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment")
                {
                    FileName = "test.pdf"
                };
                result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");
                return result;

            }

        }
        catch (Exception ex)
        {
            Response.StatusCode = (int)HttpStatusCode.BadRequest;
            return null;
        }
        Response.StatusCode = (int)HttpStatusCode.BadRequest;
        return null;

    }

如何将实际文件作为响应而不是JSON返回？我使用Postman作为测试客户端.