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 ...

An Excel file may be inaccessible due to a disk or network error, even though it isn't corrupted. In such cases, Excel won't be able to repair the file because the issue isn't with the file itself.

Microsoft Excel has an import wizard, but if you structure the text correctly, you can bypass the wizard altogether.