Solution 7: Open MS Excel in Safe Mode Solving the “ Excel cannot open because the file format is not valid ” problem can sometimes be as simple as booting your computer into Safe Mode . This is ...

With the Excel extension is not valid error, try starting the app in Safe Mode, repair the workbook or open the file in another program.

Many users reported that the issue ‘File format does not match the extension could also be resolved by unlocking the file. So, if you are not able to open the file sent as an attachment, use ...