New Post: Does ClosedXML/Open XML SDK work with xlsm files?

I need to create new XLSM file and modify existing file. I don't need to add new VBA code.

I've used the sample code and the resulting file is actually an XLSX file.

I've opened an issue:
https://closedxml.codeplex.com/workitem/9245

Is there anything I am doing wrong?

var workbook = new XLWorkbook();
var worksheet = workbook.Worksheets.Add("Sample Sheet");
worksheet.Cell("A1").Value = "Hello World!";
workbook.SaveAs("HelloWorld.xlsm");