Excel shows a warning when the file extension does not match the real file format. Microsoft names four causes, an accidental rename, a deliberate rename, a corrupted file and a file that someone renamed to mislead you. Open the file only from a trusted source. Save it with the extension that matches its real format. Repair a corrupted workbook first.
You double click a workbook and Excel warns that the file format differs from its extension. Microsoft treats the warning as a safety check.
Facts follow the Microsoft Support page on files in a different format than their extension. Microsoft limits the page to the extension mismatch warning and no other Excel open error.
What Causes the Extension Mismatch Warning?
- Accidental rename. Someone changed the extension without converting the file.
- Deliberate rename. Someone renamed the file on purpose.
- Corrupted file. Damaged contents no longer match the expected format.
- Misleading rename. Someone malicious renamed the file to trick you.
What to Check Before You Open the File?
Decide whether you trust the source. Microsoft advises you not to open the file when you cannot verify the sender. Ask yourself why the extension is wrong.
How to Fix the Warning?
| Situation | Microsoft states |
|---|---|
| Renamed by accident, trusted source | Open the file and save it with the correct extension |
| Renamed on purpose | Open the file and save it with an extension that matches its real format |
| Corrupted | Try to repair it first with the Repair a corrupted workbook page |
| Received by email | Do not open it from the mail program. Ask the sender for an .xlsb or .xls file or save the attachment and open it in Excel |
What Happens in Excel for the Web?
Excel for the web does not allow editing of non Excel files. It copies the contents to a new file and opens it in View Mode. Desktop Excel then opens the new file and shows the mismatch message.
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Frequently Asked Questions
Is the Excel Extension Warning Safe to Ignore?
No. Open the file only when you trust the source. Microsoft lists a malicious rename as one cause.
How do I Fix the Excel File Format Warning?
Open the file from a trusted source and save it with the extension that matches its real format.
What if the Workbook is Corrupted?
Try to repair it first. Microsoft links to its Repair a corrupted workbook page.
Should I Open an Excel File From Email?
No. Ask the sender to resend it or save the attachment and open it in Excel.
Key Takeaways
- Excel warns when the extension and the real format differ.
- Microsoft lists four causes including a malicious rename.
- Open files only from trusted sources.
- Save the file with the matching extension.
- Repair corrupted workbooks before you open them again.
Sources and References
Facts follow Microsoft Support.
- File format differs from extension warning in Excel (Microsoft Support) support.microsoft.com