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

Excel (.xlsx) · what this error means and every free fix to try first

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

What's actually wrong

Despite how it sounds, this often is NOT corruption: a large share of these files are a different format wearing the .xlsx extension (a CSV, an old .xls, or an HTML export from a web app). The rest are genuinely damaged containers.

Free fixes — try these first

  1. Rename a copy to .xls, .csv, or .html and try opening each — web-app 'Excel exports' are frequently HTML tables in disguise.
  2. Open Excel first, then File → Open → select the file (instead of double-clicking it).
  3. Try Open and Repair from Excel's Open dialog.
  4. Try LibreOffice Calc, which auto-detects the real format.

If the free fixes fail

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