Probably, the feature you said is "Directory Listing". It can be enabled/disabled by:
webAppContext.setInitParameter("org.eclipse.jetty.servlet.Default.dirAllowed", "true");
or
${JETTY_HOME}/etc/webdefault.xml
<servlet>
<servlet-name>default</servlet-name>
<servlet-class>org.eclipse.jetty.servlet.DefaultServlet</servlet-class>
<init-param>
<param-name>dirAllowed</param-name>
<param-value>true</param-value>
</init-param>
</servlet>
See also:
How to disable directory listing for Jetty's WebAppContext?
Jetty documentation:
http://www.eclipse.org/jetty/documentation/9.4.x/advanced-extras.html