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.

Every now and then, when I try opening an Excel file, I get a message saying that "Excel cannot open the file 'file.xlsx' because the file format or the file extension is not valid. Verify that the ...

To fix the Excel file format does not match extension error, trying opening the spreadsheet with the Open and Repair option or changing its file extension.