为什么 ModelState 在处理图像时有效 false

问：

public ActionResult Signup(UserTable u)
 {
     string fileName = Path.GetFileNameWithoutExtension(u.ImageFile.FileName);
     string extension = Path.GetExtension(u.ImageFile.FileName);
     fileName = fileName + extension;
     u.ImageUrlPath = "/Images/" + fileName;
     fileName = Path.Combine(Server.MapPath("/Images/"), fileName);
     u.ImageFile.SaveAs(fileName);

     if (ModelState.IsValid)
     {
        PasswordEncrpt EncrptedData = new PasswordEncrpt();

         u.EmailVerification = false;
         u.ActiveCode = Guid.NewGuid();
         //u.EncryptedPassword = "fff";
         u.EncryptedPassword = EncrptedData.Encode(u.SimplePassword);
         //u.LastLogin = DateTime.Now;
         u.Active = true;
         u.RegisterDate = DateTime.Now;

         Db.UserTables.Add(u);
         int a = Db.SaveChanges();
         if (a > 0)
         {
             SendMailToUser(u.Email, u.ActiveCode.ToString());

             return Json(new { success = true });
         }
     }
     else
     {
         //return Json(false);
         //return new HttpStatusCodeResult(HttpStatusCode.InternalServerError, "An error occurred");

         //var errors = ModelState.Values.SelectMany(v => v.Errors).Select(e => e.ErrorMessage).ToList();
         //return Json(new { success = false, errors });

         var errors = ModelState.ToDictionary(model => model.Key , model => model.Value.Errors.Select(e => e.ErrorMessage).ToList());

         return Json(new { success = false, errors });
     }
     return Json(false);
 }

请在 asp.net MVC EF 中使用 datebase 中的存储路径处理图像，但图像获取方式中回答这个问题

[Display（Name = “ImageUrlPath”， ResourceType = typeof（语言））] [必需（ErrorMessageResourceType = typeof（语言）， ErrorMessageResourceName = “ImageIsRequied”）]

public string ImageUrlPath {  get; set; }

[未映射]

public HttpPostedFileBase ImageFile { get; set; }