“Excel cannot open the file because the file format or file extension is not valid”

What causes this Excel error, what you can try first, and how to recover the data if the file is damaged.

Fastest fix

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Why this happens

The extension doesn't match the content (e.g. a CSV or HTML file renamed to .xlsx).
The download or email attachment was cut off, so the file is incomplete.
The file was damaged while saving (crash, full disk, sync conflict).

How to fix it

Download the file again from the original source, if possible.
In Excel use File → Open → select file → arrow next to Open → Open and Repair.
If that fails, try recovering the cell data with a repair tool — check the preview before relying on it.

Still stuck? Ask SheetFix Support