Either be honest (*) and put the actual suffix (txt, csv, xml, htm which can easily be built in Abap) or use some tool to produce an actual Excel file
Look for tool like Abap2Xls, a xml tricl as in Convert incoming XML to Excel or Excel XML - XSLT Way or use OLE2 if connected to a GUI with Excel installed.
Regards,
Raymond
(*) From the Excel security point of view...