title: Jenkins pipeline, Groovy sandbox: Interpret properties file
body: <p>I have a Jenkins pipeline step where I extract the build number from an INI file.</p>
  
  <p><a href="https://stackoverflow.com/a/38955697/18651">This answer</a> shows how to do this using a Properties object - but for this to work, I have to disable "Use Groovy Sandbox".</p>
  
  <p>How can I solve this in a "Groovy Sandbox" approved manner?</p>
  
  <p>Edit: I would prefer to use methods that are approved out of the box, rather than modifying a whitelist. So this question is really asking how to solve the problem with methods that are already allowed.</p>