How to fix "Excel cannot open the file because the file format or file…" in Microsoft Excel
Excel cannot open the file 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.
What this error means
Excel checked the first bytes and they did not look like a workbook — either the file is really another format (CSV, HTML, an old .xls) with the wrong extension, or the beginning of the file is damaged.
Why it happens
- A website exported HTML or CSV but named it .xlsx
- The file was renamed by hand
- The ZIP structure is damaged or the file is cut short
How to fix it
- Run the free check here — if it is just the wrong extension we tell you the right one, free
- Try File › Open in Excel and pick "All files", then Open and Repair
- Open it in LibreOffice Calc, which detects the real format automatically
About Excel workbook files
An .xlsx workbook is a ZIP of XML parts: each sheet is a separate file (xl/worksheets/sheet1.xml…), text cells share one string table, and xl/workbook.xml lists the sheets. When the workbook index is lost, the sheets are often still intact.