title: MULE expression-transformer not accepted
body: <p>I'm trying to learn Mule ESB but get problems with example projects. Why are these lines
  underlined red and not represented in the Message flow?</p>
  
  <pre><code>&lt;expression-transformer name="returnAttachments"&gt;
          &lt;return-argument evaluator="attachments-list" expression="*.txt,*.ozb,*.xml" optional="false"/&gt;
      &lt;/expression-transformer&gt;
  </code></pre>
  
  <p>I've cut and pasted these lines from mulesoft.org as part of a sample project.</p>