title: Serve a download of an uploaded file in Symfony2
body: <p>My Symfony2 app allows users to upload files. I'd like to users to also be able to download their files.</p>
  
  <p>If I were doing straight PHP, I'd just output the appropriate headers, then output the contents of the file. How would I do this within a Symfony2 controller?</p>
  
  <p>(If you use a hard-coded filename in your answer, that's good enough for me.)</p>