docs: function(name, url, layername, options) {
          this.layername = layername;
          OpenLayers.Layer.Grid.prototype.initialize.apply(this,
                                                           [name, url, {}, options]);
          this.extension = this.format.split('/')[1].toLowerCase();
          this.extension = (this.extension == 'jpg') ? 'jpeg' : this.extension;
      }