Excel cannot be open the file 'filename.xlsx' because the file format or file extension is not valid

Muthu69 Hi. If you have tried all possible methods to repair your Excel file but none of them work, then maybe your Excel file is not corrupted, but you just don’t have permission to open it on your system. If you don’t have permission to open the Excel file, it will display the error "Excel cannot open the file because the file format or file extension is not valid." I have tested this on my Windows 10.  You can use the following steps to grant everyone on this computer permission to open the Excel file. 

1. Right-click the Excel file that cannot be opened and select Properties.
2. After the file’s Properties dialog opens, select the Security tab, and then click the Edit button.
3. After the file’s Permissions dialog opens, click the Add button.
4. When the Select Users or Groups dialog opens, click the Advanced button. When the next dialog opens, click Find Now so all users and groups will show up in the search results list at the bottom of the dialog. Select the Everyone group from the list, and then click OK. Click OK again.
5. When you return to the Permissions dialog, you will see the Everyone group has been added to the group or user list. Select the Everyone group, check all checkboxes under Allow, and then click Apply.

That’s it. Now check if you can open the Excel file without any problems. If you want an illustrated guide, refer to this page: https://www.isumsoft.com/office/excel-cannot-open-the-file-because-extension-is-not-valid.html#way2

Please tell me if this helps. Good luck!