How to Fix a File Extension in Windows 10

On computers, the three letters after the period in the file name are known as the "file extension." The file extension explains to the operating system what type of data the file contains, and the operating system uses that information to select the application used to open the file when it is double clicked. If the extension of a file is incorrect, it may not open in the correct application. Correct the file extension to make the file open in the application of your choice when you double click it.

From the Start menu, go to the "Settings" panel. You can use "Win + I" as a keyboard shortcut if you want.

Select the "Apps" entry, then choose "Default apps" on the left sidebar. You’ll see the apps you have set as default for common tasks such as emailing, listening to music and more. Click one to change it.

To look over the complete list of file associations, scroll down and click "Choose default apps by file type." This will open a long list of file extensions, including many you probably won't recognize. You can select any entry to change its associated program.

If you select "Choose default apps by protocol," it will let you select apps to handle certain URL protocols such as Mailto and FTP. In many cases, these already change when you adjust default apps, so you normally won't have to make many changes.

If you select "Set defaults by app," you’ll see a list of all the types of files a certain app can open. This lets you easily change defaults when you notice a program opens when it shouldn’t, or vice-versa, so it's very helpful.

If you see that your file associations are messed up, you can click the "Reset" button to restore everything back to the default settings.