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 ...

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.

Fix error This file cannot be previewed because there is no previewer installed for it in Outlook while previewing Excel, PDF, Image, Word, etc. file formats.