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When Excel says it cannot open a file because its format or extension is not valid, the extension may not match the file’s actual format—or the workbook may be damaged. First verify that the file comes from a trusted source. Then check its format and try Excel’s built-in Open and Repair recovery if corruption is likely.
What the error means—and what to check first
A filename extension such as .xlsx is a clue about a file’s format, not proof of what the file contains. Microsoft notes that a file ending in .xlsx may actually be a text file that should have a .txt extension. The warning can also appear when a workbook is corrupted and Excel cannot read it. Microsoft’s format-mismatch guidance covers Microsoft 365 and Excel 2016, 2019, 2021, and 2024.
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Before trying to open a file that triggers this warning, verify its source. A misleading extension can disguise an untrusted file, so do not open it merely to test it if you cannot confirm where it came from. Ask the sender or download provider for a fresh copy from a known source.
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If you know the file was renamed
For a trusted file that was accidentally renamed, determine its actual format and use an extension that matches it. Microsoft says a known, trusted file can be opened and then saved with the appropriate extension. Do not simply change an unknown file’s name to .xlsx: renaming does not convert the file’s contents.
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If it came as an email attachment
For a trusted attachment that produces this mismatch warning, save it to your computer before opening it, or ask the sender to resend it in another Excel format, such as .xlsb or .xls. If you cannot verify the sender or attachment, do not open it; request a new copy through a trusted channel.
Recover a workbook that may be corrupted
Work on a copy of the workbook where possible, so the original remains available. Excel’s Open and Repair workflow offers two recovery choices: Repair attempts to restore as much workbook data as possible; if that does not work, Extract Data is intended to retrieve values and formulas. Microsoft’s corrupted-workbook instructions apply to Microsoft 365 and Excel 2016, 2019, 2021, and 2024.
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In Excel, select File > Open, then browse to the workbook.
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Select the workbook. Use the arrow beside Open and choose Open and Repair.
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Choose Repair first. If Excel cannot repair the workbook, repeat the process and choose Extract Data to retrieve values and formulas.
Recovery is not guaranteed. If the workbook remains unreadable, ask for a fresh copy or restore an available backup. Keeping backups or recovery copies is the practical safeguard against losing access to a damaged workbook.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Only consider the old Mac issue if the symptoms match
Microsoft documented a specific Excel for Mac issue in which the error could follow a Grant Access prompt, with Excel hanging or reporting this message. Its support page, last updated March 25, 2020, says the issue was fixed in version 16.34 or later for the listed Excel for Microsoft 365, 2019, and 2021 for Mac versions. If your problem specifically follows that prompt, update Office and check whether that behavior persists; this historical fix is not a general explanation for current format or corruption errors. Microsoft’s Mac issue note
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