SimpleGraph Excel module isn't able to create CSV file

Question

I tried to create a CSV file, but result was also a xlsx file instead, although it was named *.csv. It is possible to create such a file from its filename in a correct format or should there exist a method or parameters to create a csv file explicite (also a parameter for the delimter). Perhaps also other export formats from Excel.

public void testCreateExcelCSVTracks() throws Exception {
    ExcelSystem es = new ExcelSystem();
    Graph graph = TinkerGraph.open();
    graph.io(IoCore.graphml()).readGraph("tracks.graphml");
    if (debug)
       LOGGER.log(Level.INFO, graph.toString());

    GraphTraversalSource g = graph.traversal();
    // es.setDebug(true);
    Workbook wb = es.createWorkBook(g);
    assertEquals(3, wb.getNumberOfSheets());
    es.save(wb, "tracks.csv");
}