FromBody XmlDocument文档始终为null

Question

我不知道我是误解还是做错了什么。无论我设置的[FromBody]数据类型是字符串还是XmlDocument，它总是返回null。下面是我的代码：

        [System.Web.Mvc.HttpPost]
    [System.Web.Mvc.Route("{instance}/{customer}/Upload")]
    public async Task<ActionResult> UploadCustomer(string instance, string customer, [FromBody] XmlDocument xml)

我正在通过SoapUI实例发送请求，客户被接走。这是我正在使用的URL

http://localhost:6543/Test/AA/Customer/Upload

我的webApiConfig文件中也有这个

        public static void Register(HttpConfiguration config)
    {
        // Web API configuration and services
        config.Formatters.XmlFormatter.UseXmlSerializer = true;

        //var container = new UnityContainer();
        //container.RegisterType<IConfiguration, Configuration>();
        //config.DependencyResolver = new UnityDependencyResolver(container);
        //GlobalConfiguration.Configuration.DependencyResolver = new UnityDependencyResolver(container);

        // Web API routes
        config.MapHttpAttributeRoutes();

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }