File extensions in Windows are a legacy inherited from DOS systems and are mainly used for identify the file format. The file explorers of Microsoft’s latest operating systems usually hide them by default, but in our opinion they should be enabled for both convenience and security reasons. And users know its basic basics, operation and handling. Here we go.

How they work

Extensions are suffixes with a length of two to four placed characters after the name of the files themselves and is preceded by a period of separation. When you double-click a file, the operating system looks for the application associated with the extension that is installed on the computer (there is usually one by default) and opens the file.

A simple example. If you have text files associated with Windows Notepad, and you run a file with a .txt extension, its contents will open in that notepad. There are other ways to handle them, such as selecting the desired application when we right-click on the file, but the basic operation is as shown.

And we always talk about Windows (with legacy DOS operations that are still maintained) because other operating systems like UNIX-based macOS don’t use them for exactly the same function and aren’t as important.

File Extensions in Windows: Types

The number of extensions in Windows is huge. Some are for internal use system itself and others correspond to a wide variety of formats that a computer can handle within the various groups they are reserved for, such as image files, video, text, music, web-related files, or others.

Some of the most important:

• EXE: one of the file formats called ‘executable files’ because they are used to install applications.
• HTM/HTML – Hypertext Markup Language format for creating online web pages.
• PNG: Lossless image file format.
• JPG: another of the most widely used image formats.
• TXT: ASCII text file in plain text.
• PDF: A storage format for digital documents originally developed by Adobe.
• MP3: compressed digital audio format.
• DOC / DOCX – Microsoft Word text editor document file.
• MPEG: compressed video file.
• DLL: internal libraries for running Windows.
• COM: Applications for MS-DOS.
• REG: executable files for the Windows registry.

These are just some of the most famous examples, as there are many, some industry standards and others created by the developers themselves to run specific applications. It is worth making a safety recommendation here, which is to be careful about opening files from untrusted sources, especially those that are potentially dangerous in executable formats or that execute code such as EXE, BAT, MSI, REG and others.

How to view file extensions in Windows

Microsoft is changing its policy on whether to display file extensions in Windows. For us it is a mistake and They should always be visible to the user. Nor is it just more useful but safer. Without the file extensions displayed, it can be difficult to tell if a DOC file (for example) is actually a document and not a malicious executable.

If you don’t see them, activating them is simple. In any window, open File Explorer > View > Options > Folder Options > View and make sure the option “Hide file extensions…” is unchecked. Next, click “Apply to Folders” to make the extensions appear in all directories on your computer.

How to change the associated application

Every time you install an application on Windows that can open a certain type of file, it registers both the file extension and the software. It is perfectly possible to have multiple applications that can open the same file type. You can launch the application and then upload a compatible file to it. Or you can right-click a file to open its context menu and select one of the applications that can launch it. In the example, the image file is in JPG format:

However, each extension also has a default application associated with it. It’s an application that opens when you double-left-click on a file. On Windows, this is also the application that appears at the top of the list shown in the example above.

If you always use the same thing it is useful to assign an application that is always used by default with a specific file extension. To do this, right-click on the file and open its context menu and click “Choose another application”. Here you will see a list of those installed on your PC. Select the one you like and check “Always use this application to open files…” (in the example, those with the .jpg extension).

In Windows 10 or Windows 11, you can access the same option in System Configuration > Applications > Default applications to manage available applications.

Be careful when changing or removing extensions

You have to be careful when editing them in Windows. We have already said that the operating system depends entirely on them. If you delete or modify the extension, Microsoft systems will go crazy. For example, if you remove or modify the .jpg extension in an image file, the file will be unusablebecause Windows tries to open the file with the new associated application and cannot find it:

The solution is just a matter of overwriting the original extension, but before that Windows will warn you of the problem with an error message before making any changes.

Are there extensions on other systems?

Yes, Linux and macOS also use file extensions, but they don’t depend on them like Windows does. Instead, they use a protocol MIME where creators specify the file type. This information is stored in the file header, and both macOS and Linux use this information to determine what type of file they are working with.

Since file extensions aren’t actually necessary in macOS or Linux, it’s quite possible that you have a valid file without an extension, but the operating system can open the file with the correct program because of the file information contained in its header.

For this reason, file extensions are not displayed by default on systems such as macOS. However, you can also have Apple show the extension, and it’s probably not a bad idea for the reasons explained for Windows. The way to do this is simple, from Finder > Preferences > Advanced and enable the “Show all file name extensions” box. macOS will also display an error if you try to change the extension.