Take an Excel spreadsheet, for example. The old Excel format is .xls, but with the newer versions it is .xlsx which makes it where you cannot open the file (a trial offer may pop up asking you to try or buy) using an older Windows version.
This is a problem I experience at work a lot due to receiving email attachments from people who use newer versions. So I happened to stumble upon an easy way to convert those files to be a format I can actually open.
Using this method, you can easily convert those XLSX files, for example, to PDF, ODS (OpenDocument format), CSV (comma-separated values), TSV (Tab-separated values) and ZIP files. I found the one that works best for me is to convert to a PDF (which is opened via Adobe Acrobat Reader) as most everyone has this program already installed on their computers.
Follow these steps:
First off, while in GMail, if you have an attachment, it will be shown at the bottom of the email and you can easily see the file extension right there to help you determine if it needs converting or not (see screenshot A)
Enjoy and happy converting!