“Excel cannot open the file because the extension is not valid” error happens when you open an older version of an Excel file in a newer version. You may also encounter this error when you accidentally renamed the file using a different file format or an incorrect file extension. Also, you may get the ‘Excel cannot open the file’ error if the file has become unreadable or corrupted. In this video, you’ll learn all solutions on how to fix “Excel cannot open the file filename.xlsx because the file format or file extension is not valid error.