#targetengine "session"
// we need a targetegine to make this work
var doc = app.activeDocument; // get the current doc

// now to the event listener
app.addEventListener('afterSave', function(theEvent) {
  $.writeln('saving'); // just to see whats going on
  if (!doc.saved) {
    // catch those possible mistakes
    alert('doc was never saved');
    exit();
  }
  var aName = doc.name; // get the name
  var newName = aName.replace("indd", "idml"); // replace the indd to idml
  // crate a new File Object next to the indd
  var theFile = File(File(doc.filePath).fsName + "/" + newName);
  // export
  doc.exportFile(ExportFormat.INDESIGN_MARKUP, theFile, false);
});