title: DataAnnotation Regular Expression always returns false for file input
body: <p>I've tried so many regular expressions for the <code>RegularExpression</code> data annotation to check if the file extension is an image and it always returns false e.g. I've also tried <code>FileExtension</code> attribute but it creates an error on the jquery.validation. I'm using ASP.NET MVC 4 Razor </p>
  
  <pre><code>[RegularExpression(@"^.*\.(jpg|gif|jpeg|png|bmp)$", 
  ErrorMessage = "Please use an image with an extension of .jpg, .png, .gif, .bmp")]
  public string MyImage { get; set; }
  </code></pre>
  
  <p>and this is my markup</p>
  
  <pre><code>    &lt;div class="editor-field"&gt;            
          @Html.TextBoxFor(x =&gt; x.DepartmentImage, new { type = "file" })            
          @Html.ValidationMessage("DepartmentImageError")
          @Html.ValidationMessageFor(model =&gt; model.DepartmentImage)
      &lt;/div&gt;
  </code></pre>
  
  <p>Could someone show me how to make it work?</p>