the file format or file extension is not valid.
Excel error message: the file format or file extension is not valid. Verify that the file has not been corrupted and that the file matches the format of the file
iMac 21.5", macOS 10.14
.xls
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Did you get a copy of the current, free LibreOffice from the link I provided earlier, and open the .xls file with it. If that works, you can save it as .xls or .xlsx, and then it will open again in Excel or Numbers.
Because LibreOffice is obtained from its Developer site, you will have to undergo a one-time ritual to satisfy macOS security. Once you have installed it, visit the Applications folder, right-click on LibreOffice, and just select Open. Apple's Gatekeeper will verify the application with a progress bar, and then you will receive a dialog with another Open button. Click that too.
LibreOffice will start, and subsequent usage is the normal double-click (or single-click in Launchpad, or Dock) to launch it. You will want LibreOffice Calc from the initial application panel, as that is the Excel clone.
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If you turn on Optimize storage on iCloud Drive, older, little used docs will be moved to cloud if you need the space on your drive.
the file format or file extension is not valid.