ted from the url, based on the
      url string ending, as follows
      <ul>
      <li>.jpg, .gif, .png => Image</li>
      <li>.txt, .xml, .css => Text</li>
      <li>.swf => Swf</li>
      <li>.flv, .f4v => Video</li>
      <li>.mp3 => Audio</li>
      <li>any other alphabetic ending, e.g. .abcde, => Text</li>
      <li>otherwise, e.g. no ending, => Error is thrown.</li></ul>
      <p>
      At instantiation, the factory is given an IAssetTypeMap instance.  Given the asset type, this map 
      is used t