How to disable a ribbon button when a folder is selected

I have added a new button to the SharePoint ribbon for document libraries. This all works well and the button is enabled / disabled depending on the number of items selected... fine!

I want to improve this by disabling the button when a folder is selected similar to how the standard SharePoint 'email a link' button works. Can anyone offer any advice on how to approach this? I have Googled around, but have not been able to find anything.

The code I currently have is as follows:

  <CustomAction      Id="Muhimbi.SharePoint.DocumentConverter.PDF.Ribbon.Documents.Copies.Controls.ConvertAndDownload.Action"      Location="CommandUI.Ribbon"      RegistrationType="ContentType"      RegistrationId="0x0101"      >      <CommandUIExtension>        <CommandUIDefinitions>          <CommandUIDefinition            Location="Ribbon.Documents.Copies.Controls._children">            <Button Id="Muhimbi.SharePoint.DocumentConverter.PDF.Ribbon.Documents.Copies.Controls.ConvertAndDownload.Button"                    Command="Muhimbi.SharePoint.DocumentConverter.PDF.Ribbon.Documents.Copies.Controls.ConvertAndDownload.Button.Command"                    Image16by16="/_layouts/images/Muhimbi.PDFConverter/pdf16.gif"                    Image32by32="/_layouts/images/Muhimbi.PDFConverter/pdf32.gif"                    LabelText="$Resources:ecb_title;"                    Sequence="12"                    TemplateAlias="o1" />          </CommandUIDefinition>        </CommandUIDefinitions>        <CommandUIHandlers>          <CommandUIHandler            Command="Muhimbi.SharePoint.DocumentConverter.PDF.Ribbon.Documents.Copies.Controls.ConvertAndDownload.Button.Command"            CommandAction="javascript:window.location='{SiteUrl}/_layouts/Muhimbi.PDFConverter/Convert.aspx?action=ConvertAndDownload&amp;ListId={ListId}&amp;ItemId=' + SP.ListOperation.Selection.getSelectedItems()[0].id + '&amp;Source=' + window.location"            EnabledScript="javascript:function singleEnable()            {              var items = SP.ListOperation.Selection.getSelectedItems();              var ci = CountDictionary(items);              return (ci == 1);            }            singleEnable();" />        </CommandUIHandlers>      </CommandUIExtension>    </CustomAction>  Answers: 

Use the fsObjType property:
EnabledScript="javascript:    var items = SP.ListOperation.Selection.getSelectedItems();    (items.length == 1 && items[0].fsObjType == 0);"