title: ASP.NET MVC FilePathResult: How to return an html file not found?
body: <p>The following code is hopefully a correct way to return an image that exists on disk using ASP.NET MVC 3:</p>
  
  <pre><code>public FilePathResult GetThumbnail(string imageName)
  {
      if( !String.IsNullOrEmpty(imageName) &amp;&amp;
          Regex.IsMatch(imageName, @"^p\d{10}.jpg$"))) ) // p0000000000.jpg
      {
          var homePath = Server.MapPath("~/Content/previews");
          var imagePath = Path.Combine( homePath, imageName );
  
          if( System.IO.File.Exists(imagePath) )
              return this.File(imagePath, "image/jpeg");
      }
  
      return ???   
  }
  </code></pre>
  
  <p>If you don't find the file, what could you return that would represent an HTML 404 error (or equivalent?)</p>