title: How does symfony choose the very content type?
body: <pre><code>  txt:  text/plain
    js:   [application/javascript, application/x-javascript, text/javascript]
    css:  text/css
    json: [application/json, application/x-json]
    xml:  [text/xml, application/xml, application/x-xml]
    rdf:  application/rdf+xml
    atom: application/atom+xml
  </code></pre>
  
  <blockquote>
    <p>It is used by the framework to
    automatically manage the Content-Type
    of the response, based on the request
    URI extension.</p>
  </blockquote>
  
  <p>As above,there are 3 different content types for <code>.js</code> extension,how does symfony choose the final content type?</p>