title: Rails 3.1: Determine if asset exists
body: <p>Is there a built-in way to determine if an asset exists without resorting to <code>File.exists?(File.join(Rails.root, "foo", "bar", "baz"))</code> and that looks through the asset paths.</p>
  
  <p>My app goes and fetches images from a remote server on a Resque queue; until we have the image downloaded I want to serve a placeholder image.  Currently I'm using <code>File.exists</code>... but this means hard-coding a path, which sucks, or looking through the configured asset paths.  It seems like this should be there already, but I can't find it in the docs.</p>