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You cannot open the Excel file and receive the following error message:
"Excel cannot open the file 'filename.xlsx' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file."

Error Message when open Excel file

Note that this is a method specific to the above error message.


How to do:

Fix "Excel cannot open the file because the file format or file extension is not valid"

 

Step 1: Find the corrupt Excel file, press F2 or right-click and choose Rename. Then change the file extension from .xlsx to .xls (tif he file extension is hidden, go to Folder Options to show the file extension).

Change to .xls 

 

Step 2: After that open Excel file (.xls). You should now be able to open the normal Excel file.
However, for safety and better use, please convert the file extension from.xls to .xlsx again.


Step 3: Click File, then choose Convert to convert .xlsx. Click OK and Yes to agree to convert.

Convert function 

Excel file has been fixed and converted to its original format (.xlsx)

Done, now you can reopen the Excel file (.xlsx) to use it.


Conclusion:
This is an extremely fast method to handle the error in Excel: “because the file format or file extension is not valid”.
I'm not 100% sure this method solved. But if this doesn't work, you can comment below, I will help you.

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