Build 3.5.0.85 of xlsgen makes it possible to define alternate filename associations for reading or writing files using xlsgen.
It works like this. If you have a file ending with .aaa and you know this file is an Excel 2007 file, you can use the following in order to make sure that xlsgen will open it correctly :
engine.Workbooks.AddFilenameAssociation("aaa", xlsgen::excelversion_2007);
and then,
workbook = engine.Open("inputfile.aaa", "outputfile.xlsx");
This works with New(), Open() and other methods where an Excel filepath is passed.