无法在web api c#中接收表单数据

我试图使用XMLHttpRequest将上传的excel文件发送到web api，但我在web api中收到的所有文件都是一个对象。如何在web api中接收上传的文件?

 upload(file: File): void {
        let formData: FormData = new FormData(),
        xhr: XMLHttpRequest = new XMLHttpRequest();
        formData.append("uploads", file, file.name);
        xhr.open('POST', this._expenseServiceUrl + 'expenses' + '/' + 'massupload', true);
        xhr.send(formData);
}

Web API

        [Route("massupload")]
        [HttpPost]
        public HttpResponseMessage MassUpload([FromUri] dynamic uploads)
        {
            try
            {
                response = Request.CreateResponse(HttpStatusCode.OK, "");
            }
            catch (Exception exception)
            {
            }
            return response;
        }

foreach (string fileId in Request.Files)
{
   HttpPostedFileBase file = Request.Files[fileId] as HttpPostedFileBase;
}