title: search for a specific file type with ack
body: <p>How can I use ack to search in files of a specific file type.</p>
  
  <p>For example</p>
  
  <pre><code>ack -f .scss blah
  acl -f .rb blah
  </code></pre>
  
  <p>Obviously the above does not work but how can I do this with ack?</p>