<form method="post" enctype="multipart/form-data" action="/api/api.asmx/UploadFile">
  <input type="text" name="fileName" value="" />
  <input type="file" name="fileBody" />
</form>

    [WebMethod(Description = "lololo")]
    public void UploadFile(string fileName)
    {
        System.Web.HttpPostedFile file = HttpContext.Current.Request.Files["FileBody"];
        string path = HttpContext.Current.Server.MapPath("~/App_Data/");
        path += file.FileName; //тут дает ошибку: System.NullReferenceException: Object reference not set to an instance of an object.
        file.SaveAs(path);
    }