The XLSX format is a Microsoft Excel 2007 format that is not compatible with older versions of the said application. Luckily, there are alternatives to converting a 2007 Excel document to an older version of it. I prefer the online way but there is also a compatibility pack for MS Office that does the trick. The format is currently not supported by GMail attachment viewer or any of the online office suites like Google Docs so you have no choice but to have it converted to an old format else you will not be able to view it.

However, you can easily convert .xlsx spreadsheet to the traditional .xls format using the Microsoft Office compatibility Pack available here. This will help you open, edit, and save .xlsx files in Office 2000, Office XP, or Office 2003. More details on Office 2007 FAQ.

Alternatively, you can convert .xlsx to .xls or other text formats online using the Zamzar.com service here. Unlike the Microsoft compatibility pack, the Zamzar .xlsx to .xls conversion services will not require a copy of Microsoft Office on your computer.

And if you have OpenOffice, Zamzar can also convert the XLSX file to OpenDocument spreadsheet format.