If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.

How to Open Multiple CSV Files in One Excel Spreadsheet. The CSV file extension appears on text documents that contain comma-separated values. Like spreadsheet data, each line in a CSV file ...

However, let’s assume that you want to send some Excel spreadsheets to someone who has an old version of Excel. In this case, you must save the file in .xls format instead of .xlsx.