‘File Format and Extension Don’t Match’ Error Fixed in Excel

The ‘File Format and Extension of Don’t Match’ error appear when the user tries to open certain files in Excel. Even if the user clicks on Yes to open it, all previously saved information on that file will be gone.

“The file format and extension of don’t match.” Excel Error

What’s causing the ‘File Format and Extension of Don’t Match’ error?

  • Incorrect extension – It the majority of cases, this particular issue will occur because the Excel file you’re trying to open is in fact of a different extension than the one that’s currently hardwired. This can happen automatically after the file is converted or manually after a user intervention. If this scenario is applicable, you can fix the issue by changing the extension manually to the correct one.
  • Excel file is blocked – If you obtained the Excel file after downloading it from an email attachment, there’s a high chance that the file is blocked at a Properties level. Nowadays, a lot of email providers will block it automatically for security purposes. In this case, you can resolve the issue automatically by unblocking the file via the Properties menu.
  • The file is incompatible with Excel – As it turns out, this issue can also occur due to an incompatibility between the file you’re encountering the issue on and Excel. To fix this problem, you can rely on another 3rd party tool equivalent like OpenOffice Calc (belonging to the OpenOffice suite).
  • Protected Views is Enabled – There’s also a new security option that might end up preventing the main excel application from opening certain files that we obtained through an email attachment. In this case, you can resolve the issue by accessing your Excel settings and disabling Protected Views from the Trust Center Menu.
  • The message is not suppressed – If you mind the message but you don’t want to update your Office build to a newer version in which this issue was resolved, one viable way of ensuring that the error doesn’t return is to suppress the warning message by using the Registry Editor to create an ExtensionHardening value.

Method 1: Changing the extension manually

In most cases, the error message is pointing to the right cause. If you see the ‘File Format and Extension of Don’t Match’ error, the Excel file you are trying unsuccessfully to open is likely in fact of a different extension that the one that is currently hardwired.

Several affected users that were also encountering this issue have reported that they managed to fix the problem by manually changing the extension (via renaming) to various popular Excel formats until they found the correct one.

Here’s a quick guide on how to do this:

  1. First things first, open File Explorer and click on the View tab at the top of the screen. From the newly appeared vertical menu, ensure that the box associated with File Name Extensions is checked.
  2. Once file Name extensions have been enabled, navigate to the location where you store the excel file that is showing this error.
  3. When you get there, right-click on it and choose Rename from the context menu.
  4. Then, modify the extension (after ‘.’) systematically to  .xls  then  .xlsx   then  .xlsm, and attempt to open the Exel file after each modification.
  5. Eventually, you should stumble upon the correct format that will open the file without triggering the same error message.
    Modifying the Excel file to the correct format

    In case none of the formats above work or you’re looking for a different fix for the issue, move down to the next method below.

Method 2: Unblocking the file (if applicable)

As it turns out, the ‘File Format and Extension of Don’t Match’ error can also occur because it’s being blocked at a Properties level. This is a typical occurrence with files downloaded over the internet or received via email attachments as a security option.

If this scenario is applicable, you can resolve the issue very easily by accessing the Properties screen of the file that is triggering the error message and unblocking it via the Security tab.

Here’s a quick guide on unblocking excel files that are triggering ‘File Format and Extension of Don’t Match’ errors at launch:

  1. Navigate to the location where the Excel file is stored and right-click on it. Then, click on Properties from the newly appeared context menu.
    Properties of Excel File
  2. Once you’re inside the Properties menu, select the General tab from the horizontal menu at the top, then scroll down to the Security section and simply click the Unblock button.
    Unblocking the file
  3. As soon as the file has been unblocked, launch the file again (no restart required) and see if the issue is now resolved.

If the same ‘File Format and Extension of Don’t Match’ error is still appearing, move down to the next potential fix below.

Method 3: Opening the file with a 3rd party equivalent

Several affected users that were also encountering this issue have confirmed that they were finally able to open the Excel file and edit without encountering the ‘File Format and Extension of Don’t Match’ error by installing a free Excel equivalent and using that to open the problematic file.

As it turns out, this operation might end up avoiding corrupted instances that ultimately make the file unopenable. And if you decide on using OpenOffice, it’s notoriously known for being way better at eliminating corrupted attachments from Excel files when migrating them to the .ODS format.

Here’s what you need to do to open the file with a 3rd party equivalent:

  1. Visit this link (here) from your favorite browser and click on the Download section. Next, select Windows (Exe), your native language and select the latest available version before clicking on the Download Full installation button.
    Downloading the Full installation button
  2. Once the download is complete, open the installation executable and follow the on-screen prompts to complete the unpacking of the installation.
    Unpacking the OpenLibre installation
  3. When the main installation window is open, select Custom from the list of available options, then disable every other Program Module aside from OpenOffice Calc.
    Installing OpenOffice Calc
  4. Click Next, then wait for the installation to complete.
  5. After OpenOffice Calc was successfully installed, right-click on the file that you’re having issues with and choose Open with > OpenOffice Calc.
  6. See if 3rd party application can open the file without encountering the same error message.

If the ‘File Format and Extension of Don’t Match’ error is still appearing or you’re looking for a different fix that doesn’t require the installation of other 3rd party components, move down to the next method below.

Method 4: Disabling Protected View

As it turns out, the ‘File Format and Extension of Don’t Match’ error will often occur because a fairly new security option (Protected View) is preventing the Excel application from opening up certain files that were obtained through email attachments.

If this scenario is applicable, you can bypass this security option by accessing your Excel settings menu and disabling the Protected View feature altogether. Several affected users that were also affected by this issue have confirmed that the issue was completely resolved.

Here’s  a quick guide on how to disable Protected View from your Excel settings:

Note: The steps below should work for every recent Excel version (2010 and above).

  1. Open Excel and then click on File from the ribbon bar at the top of the screen. Next, from within the File menu, go to Options at the bottom of the vertical menu on the left-hand side pane.
  2. After you’re inside the Excel Options menu, select the Trust Center tab from the left-hand side pane.
  3. Once you manage to get inside the Trust Center menu, move over to the right-hand pane and click on Trust Center Settings.
  4. From inside the Trust Center menu, select Protected Views from the left-hand pane. Next, turn your attention towards the right pane and uncheck every box that’s associated with every Protected View condition. After you do this, click Ok to save the changes.
  5. As soon as you manage to enforce the changes, restart your Excel application and open the presentation window that was previously triggering the ‘File Format and Extension of Don’t Match’ error.
Disabling protected views

If the same File Format and Extension of Don’t Match error is still occurring, move down to the next method below.

Method 5: Suppressing the warning message

If none of the methods above have allowed you to fix the File Format and Extension of Don’t Match error message, and you don’t want to update your Office installation to a newer version where the issue is resolved, you also have the option to create a registry key that is capable of suppressing the warning error message altogether.

But before you do this, keep in mind that going this route might leave your system vulnerable to exploits in the future. But in case the affected computer is part of an organization network, it should not affect you in any way.

Here’s a quick guide on suppressing the File Format and Extension of Don’t Match error message by using Registry Editor:

  1. Press Windows key + R to open up a Run dialog box. Once you’re inside the Run box, type ‘regedit’ and press Enter to open up Registry Editor. When you’re prompted by the UAC (User Account Control), click Yes to grant administrative privileges
    Regedit Command
  2. Once you’re inside the Registry Editor utility, use the right-hand pane to navigate to the following location:
    HKEY_CURRENT_USER\Software\Microsoft\Office\*X*\Excel\Security

    Note: You can also paste the location directly in the navigation bar and press there to get there instantly. Also, keep in mind the X is simply a placeholder – Replace it with the current version of your Office installation.

  3. After you manage to get to the correct location, move over to the right-hand pane, then right-click on an empty space and select NEW > Dword (32-bit) value.
    Creating the 32-bit value
  4. After the new Dword value has been created, name it ExtensionHardening. Then, double-click on it and set the Base to Hexadecimal and the value to 0 before clicking on.
    Creating the ExtensionHardening value
  5. Once the change has been made, close the Registry Editor and restart your computer. At the next system startup, you should no longer encounter the same error message.

Kevin Arrows


Kevin is a dynamic and self-motivated information technology professional, with a Thorough knowledge of all facets pertaining to network infrastructure design, implementation and administration. Superior record of delivering simultaneous large-scale mission critical projects on time and under budget.

Expert Tip

‘File Format and Extension Don’t Match’ Error Fixed in Excel

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