I doubt it still works but things I needed to get through email I would embed in word documents.
I pity the programmer that has to decode the 1000 versions of xls to find the binary blob that could be a virus.
Alas, it's more difficult to get excel to accept that it shouldn't delete leading zeros than it is to check a spreadsheet's sus-o-scale.
xls morphed with every version of Microsoft Excel. MS Excel has pretty good backwards compatibility, but making an xls parser is notoriously hard because of the many differences between versions.