title: Find the extension of a filename in Ruby
body: <p>I'm working on the file upload portion of a Rails app.  Different types of files are handled differently by the app.</p>
  
  <p>I want to make a whitelist of certain file extensions to check the uploaded files against to see where they should go.  All of the file names are strings.  </p>
  
  <p>I need a way to check only the extension part of the file name string.  The file names are all in the format of "some_file_name.some_extension".</p>