I need to export multiple data tables to Excel on the clients machine, each to their own sheet. If it was just one sheet, I'd use the Excel/csv content type, but I've heard something about an XML format that can represent an entire workbook. I don't want to go down the Packaging and .xlsx route, so I need standard .xls.
Our bug tracker, Gemini, used to have an export function that produced an XML file that Excel automatically opened as a multi-sheet workbook, but I can't find it. Is there still such a mechanism, and where can I find that schema?