title: FileExtensionsAttribute and constant Extensions
body: <p>I am having issues with the FileExtensionsAttribute. The validation works just fine and all. But I would like to alter the Extensions property of FileExtensionsAttribute dynamically. The problem is that the property only accepts constant values.</p>
  
  <p>I want to achieve something like:</p>
  
  <pre><code>[Required]
  [FileExtensionsAttribute(Extensions = MyStringVariable)]
  public HttpPostedFileBase Files { get; set; }
  </code></pre>
  
  <p>How can I do this? Is the only way to create a custom attribute for this? It seems that it's not possible to subclass FileExtensions because of the fact that it is sealed.</p>
  
  <p>Thanks</p>