asp.net core 生成文件下载

public IActionResult down(int id, string filename)
        {
            string path = hostingEnvironment.WebRootPath + "/ContractFiles/" + id + "/" + filename;

           
            var contentType = "APPLICATION/octet-stream";
            
            FileStream fs = new FileStream(path, FileMode.Open);
            return File( fs, contentType, filename);
            
        
        }