title: Why is RemoteFileTemplate ignoring the value set in setFileNameExpression?
body: <p>Using Spring-integration 4.1.1.RELEASE to upload files in message payloads to remote sFtp shares. I'm using an sFtpRemoteFileTemplate initialized like this:</p>
  
  <pre><code>Expression remoteDirectoryExpression = new  LiteralExpression("si.sftp.sample");
  SpelExpressionParser parser = new SpelExpressionParser();
  Expression fileNameExpression = new LiteralExpression("foo.txt");
  
  template = new SftpRemoteFileTemplate(sessionFactory);
  template.setCharset("UTF-8");
  template.setBeanFactory(beanFactory);
  template.setAutoCreateDirectory(true);
  template.setRemoteDirectoryExpression(remoteDirectoryExpression);
  template.setFileNameExpression(fileNameExpression);
  template.setTemporaryFileSuffix(".writing");
  template.setUseTemporaryFileName(true);
  template.afterPropertiesSet();
  </code></pre>
  
  <p>However, whatever literal I place in the fileNameExpression is ignored, and the uploaded file will always be named <strong>the same as the original file</strong>. The result is always the same as if it were:</p>
  
  <pre><code>Expression fileNameExpression = parser.parseExpression("payload.name");
  </code></pre>
  
  <p>Why is that? Am I missing something?</p>