Built a custom search index to crawl a media library as described here:

http://devnet.kentico.com/Videos/System-Management/Smart-Search-in-Kentico-CMS-6.aspx

However, I slightly modified it to store the file extension as well via CMS.SiteProvider.SearchHelper.AddField(). While I'm able to successfully search against the index via the standard Smart Search web parts, how would I go about adding a filter allowing me to return only results matching my custom parameter, i.e. fileextension = 'pdf'?