title: Rails 5.0.7.2 MIME::Type Class has no method `lookup`
body: <p>I am running Rails 5.0.7.2. In my project I have so far always set the mime type like follows:</p>
  <pre><code>render :js =&gt; File.read(js_file), :content_type =&gt; 'application/javascript'
  </code></pre>
  <p>(for example). It appears, however, I shouldn't be doing it this way but by using the Class MIME::Type. On Rails console I can find it and print its methods. I used <code>(MIME::Type.methods - Object.methods).sort</code>. It only gives me three methods: <code>match</code>, <code>simplified</code> and <code>i18n_key</code>. The documentation says there should be methods <code>lookup</code> and <code>lookup_by_extension</code>.</p>
  <p>When I try using MIME::Type.lookup('application/javascript') in my code I get the same error: as if the method wasn't defined.</p>
  <p>Does anybody know what is the problem? Thanks in advance!</p>