How to fix file format and extension of Microsoft Office Excel Document Don’t Match The file could be corrupted or unsafe warning?

When trying to open certain files in Microsoft Office, especially Microsoft Excel, users may encounter an error prompt when the file format and extension of the document do not match. If the “Yes” button is clicked, any previously saved data will be lost.

In order to repair corrupted Excel spreadsheets, the user has the option of utilizing the last backup or restoring to the previous stable copy of the file. Alternatively, Recovery Toolbox for Excel can be used, as it has a high probability of returning .xlsx files to their original state before being damaged.

Solution 1: Change File Extension Manually

1. Open File Explorer > View tab > Check File name extensions option.
2. Right-click on the file and choose Rename from the context menu.
3. Modify the extension to .xls, .xlsx or .xlsm, and attempt to open the Exel file after each rename.

Solution 2: Unblock at File Properties level

1. Right-click the file and choose Properties from the context menu.
2. Select the General tab then scroll down to the Security section and click the Unblock button.

Solution 3: Open via 3rd Party Equivalent Software

1. Download the Full installation of OpenOffice.
2. Double-click the installation executable and follow the on-screen prompts to complete the installation.
3. After OpenOffice Calc was installed, right-click on the file that you’re having issues with and choose Open with > OpenOffice Calc.

Solution 4: Disable Protected View

1. Open Microsoft Excel > File > Options.
2. Select the Trust Center tab > Trust Center Settings.
3. Select Protected Views, uncheck all Protected View condition option then click Ok to save the changes.
4. Restart Microsoft Excel.

Solution 5: Disable Add-ins

1. Hold CTRL key and click on application icon till it asks you for Safe mode confirmation.
2. If the problem does not happen again, then start application normally and disable add-ins one by one and start application again every time you disable an add-in to determine the culprit add-in.
3. To disable the add-in, go to File > Options > Add-ins > Click on Go button beside Manage: Excel Add-ins > Untick the Add-ins to disable and tick to enable them.

Solution 6: Suppressing Warning Message using Registry Key

1. Press Windows + R to open the Run dialog box.
2. Type regedit and press Enter to open Registry Editor. If the UAC (User Account Control), click Yes to grant administrative privileges.
3. Go to the following location: HKEY_CURRENT_USER\Software\Microsoft\Office\*X*\Excel\Security
   • For Excel 2007: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Security
   • For Excel 2010: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Excel\Security
   • For Excel 2013: HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Excel\Security
   • For Excel 2016: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Security
4. Right-click on an empty space and select NEW > Dword (32-bit) Value.
5. Name it ExtensionHardening, double-click on it and set the Base to Hexadecimal and the value to 0.
6. Close the Registry Editor and restart the computer.