What Is a File Extension?

File Extensions, Extensions vs Formats, Executable Extensions, & More

Screenshot of six files and their file extensions in Windows

A file extension sometimes called a file suffix or a filename extension is the character or group of characters after the period that makes up an entire file name.

The file extension helps an operating system, like Windows, determine which program on your computer the file is associated with.

For example, the file myhomework.docx ends in docx, a file extension that might be associated with Microsoft Word on your computer. When you attempt to open this file, Windows sees that the file ends in a DOCX extension, which it already knows should be opened with the Microsoft Word program.

File extensions also often indicate the file type, or file format, of the file... but not always. Any file's extensions can be renamed but that won't convert the file to another format or change anything about the file other than this portion of its name.

File Extensions vs File Formats

File extensions and file formats are often spoken about interchangeably - we do it here on this website, too. In reality, however, the file extension is just whatever characters are after the period while the file format speaks to the way in which the data in the file is organized - in other words, what sort of file it is.

For example, in the file name mydata.csv, the file extension is csv, indicating that this is a CSV file. I could easily rename that file to mydata.mp3 but that wouldn't mean that I could play the file on my smartphone. The file itself is still rows of text (a CSV file), not a compressed musical recording (an MP3 file).

Changing the Program That Opens a File

As I already mentioned, file extensions help Windows, or whatever operating system you're using, determine which program is to open those types of file, if any, when those files are opened directly, usually with a double-tap or double-click.

Many file extensions, especially those used by common image, audio, and video formats, are usually compatible with more than one program you have installed.

However, in most operating systems, only one program can be set to open when the file is accessed directly. In most versions of Windows, this can be changed via settings found in Control Panel.

Never done this before? See How to Change a Default Program for a Specific File Extension for detailed instructions on changing what program opens files with certain file extensions.

Converting Files From One Format to Another

Like I mentioned above in File Extensions vs File Formats, simply renaming a file to change its extension won't change what type of file it is, even though it might appear as though that happened when Windows shows the icon associated with the new file extension.

To truly change the type of file, it has to be converted using a program that supports both types of files or a dedicated tool designed to convert the file from the format it's in to the format you want it to be in.

For example, let's say you have an SRF image file from your Sony digital camera but a website you want to upload the image to only allows JPEG files. You could rename the file from something.srf to something.jpeg but the file wouldn't really be different, it would only have a different name.

To convert the file from SRF to JPEG, you would find a program that fully supports both so you could open the SRF file and then export or save the image as JPG/JPEG. In this example, Adobe Photoshop is a perfect example of an image manipulation program that could do this job.

If you don't have access to a program that natively supports both formats you need, many dedicated file conversion programs are available. I highlight a number of free ones in our Free File Converter Software Programs list.

Executable File Extensions

Some file extensions are classified as executable, meaning that when clicked, they don't just open for viewing or playing. Instead, they actually do something all by themselves, like install a program, start a process, run a script, etc.

Because files with these extensions are just a single step away from doing lots of things to your computer, you have to be very careful when you receive a file like this from a source you don't trust.

See our List of Executable File Extensions for file extensions to be extra cautious about.