Hi,<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
Currently you can open .xltx and .xltm format file with Aspose.Cells for .net. Just like:
string infn = @"E:\ test\Book11.xltx";
Workbook book = new Workbook();
book.Open(infn); //or book.Open(infn, FileFormatType.Excel2007Xlsx);
But it is not supported to save to .xltx and .xltm format yet, you may save them to .xlsx or .xlsm format.
book.Save(outfn, FileFormatType.Excel2007Xlsx); //or book.Open(infn, FileFormatType.Excel2007Xlsm);
We will soon provide a version that support to save to .xltx and .xltm format. Thank you for your patience.