I have a web page which links to an Excel 2007 worksheet. It is a .xls file and not .xlsx file. When I click on the link I get the usual dialog box
.xls
.xlsx
If you're using the handy cfcontent trick to output as an excel file, consider using cfx_query2excel, it will write real excel files that should get you past this. It's a Java library and was worth it.
Best of luck.