MVC中ModelState不识别已注册合法文件上传扩展名问题

我创建了一个联系表单，相关代码如下：

视图代码 #

@model LessonsLearned.Domain.Common.ViewModels.ContactFormViewModel

@using (Html.BeginForm("SubmitForm", "Help", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
<p>@LLResources.GetResourceText("Question")</p>
<br />
<div class="row form-group">
   <div class="col-12">
      <b>@LLResources.GetResourceText("Admin_Email")</b>
   </div>
   <div class="col-12">
      @Html.EditorFor(model => model.Email, new { htmlAttributes = new { @class = "form-control", autocomplete = "off" } })
   </div>
</div>
<div class="row form-group">
   <div class="col-12">
      <b>@LLResources.GetResourceText("General_Name")</b>
   </div>
   <div class="col-12">
      @Html.EditorFor(model => model.Name, new { htmlAttributes = new { @class = "form-control", autocomplete = "off" } })
   </div>
</div>
<div class="row form-group">
   <div class="col-12">
      <b>@LLResources.GetResourceText("Admin_SchoolName")</b>
   </div>
   <div class="col-12">
      @Html.EditorFor(model => model.SchoolName, new { htmlAttributes = new { @class = "form-control", autocomplete = "off" } })
   </div>
</div>
<div class="row form-group">
   <div class="col-12">
      <b>@LLResources.GetResourceText("Message")</b>
   </div>
   <div class="col-12">
      @Html.EditorFor(model => model.Message, new { htmlAttributes = new { @class = "form-control", autocomplete = "off" } })
   </div>
</div>
<div class="row form-group">
   <div class="col-12">
      <b>@LLResources.GetResourceText("Observation_Attachments")</b>
   </div>
   <div class="col-12">
      @Html.TextBoxFor(m => m.File, new { type = "file" })
      @Html.ValidationMessageFor(m => m.File)
   </div>
</div>
<div class="row">
   <div class="col-12">
      <input type="submit" value="Submit" class="btn btn-success float-md-right mr-md-2" />
   </div>
</div>
}

视图模型代码 #

public class ContactFormViewModel
{
    [Required(ErrorMessage = "Email address is required.")]
    [EmailAddress(ErrorMessage = "Must be a valid email address.")]
    public string Email { get; set; }
    [Required(ErrorMessage = "Name is required.")]
    public string Name { get; set; }
    [Required(ErrorMessage = "School name is required.")]
    public string SchoolName { get; set; }
    [Required(ErrorMessage = "Message is required.")]
    [DataType(DataType.MultilineText)]
    public string Message { get; set; }
    [FileExtensions(Extensions = "jpg, jpeg, png")]
    public HttpPostedFileBase File { get; set; }
}

控制器提交方法 #

[HttpPost]
public ActionResult SubmitForm(ContactFormViewModel contactFormViewModel)
{
    if (ModelState.IsValid)
    {
        //Do stuff
        ViewBag.result = "Form submitted successfully!";
        return View("Index", new ContactFormViewModel());
    }
    else
    {
        //Don't do stuff
        ViewBag.result = "Form was not submitted, please fix errors.";
        return View("Index", contactFormViewModel);
    }

    return RedirectToAction("Index");
}

但当上传jpg或png格式的文件并提交表单时，ModelState始终提示错误：The File field only accepts files with the following extensions: .jpg, .jpeg, .png，确认上传的确实是jpg或png文件，请问为何无法识别？

问题出在[FileExtensions]特性的使用细节上：

• 扩展名列表格式错误：FileExtensions特性要求扩展名之间不能有空格，你当前的写法是"jpg, jpeg, png"，逗号后的空格会让验证逻辑把 jpeg（带空格）和 png（带空格）判定为合法扩展名，但实际上传文件的扩展名是不带空格的，因此验证失败。

修改视图模型中的特性，去掉扩展名之间的空格即可：

[FileExtensions(Extensions = "jpg,jpeg,png")]
public HttpPostedFileBase File { get; set; }

额外补充：FileExtensions特性仅检查文件名后缀，不会验证文件实际内容。如果需要确保上传的是真实图片，可在控制器中添加MIME类型验证：

if (contactFormViewModel.File != null)
{
    var allowedMimeTypes = new[] { "image/jpeg", "image/png" };
    if (!allowedMimeTypes.Contains(contactFormViewModel.File.ContentType))
    {
        ModelState.AddModelError("File", "Only JPG/PNG images are allowed.");
    }
}

内容的提问来源于stack exchange，提问作者Hannah Hayes