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What is a Filename Extension?

By G. Wiesen
Updated May 16, 2024
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A filename extension is part of a computer filename that has traditionally been used to indicate the type of file as well as potential programs that may be used to open or run that file. The system used for extensions to filenames usually depends on the operating system (OS) a person is using, and some types of OS do not use extensions on filenames at all. Extensions are typically identified as the last part of a file name, and are usually separated from the file name by a dot. A filename extension is commonly short, often only three letters, and is often used as a name to refer to a particular type of file.

Sometimes also called a file type, a filename extension is typically used by the OS on a computer to properly recognize different types of files. This was done exclusively in certain types of OS in the past, though newer kinds of OS often have the ability to recognize different files through data included in the file itself. Newer types of OS will still often use filename extensions for computer files, however, to ensure backward compatibility and to remain familiar to older computer users. These systems are sometimes unnecessary, however, and a filename extension can potentially be used for malicious reasons as well.

One of the most common forms of filename extension has been a system often referred to as “8.3” or “basename.ext.” Older types of computer OS would often only allow a maximum of eight characters for the name of the file, which was followed by an extension of three characters. This is the source of the 8.3 name, as well as the “basename.ext” name, which includes a base name of only eight characters and “.ext” to indicate a three character extension. There are many different popular and common types of filename extension such as “.txt” for text files, “.jpg” for JPEG or image files, and “.mp3” for MPEG version three files.

Certain types of OS do not require a filename extension for a file, and often identify files by data included elsewhere in the file. There are also extensions of different sizes such as “.html” for hypertext markup language (HTML) files. Modern systems typically allow users to indicate what program is used to open certain files, both as a default and by choosing while opening a file.

Certain types of programs have hidden extensions from users, and so a virus could be created with a name such as “basename.txt.EXE” and only “basename.txt” would be displayed. This would appear like a harmless text file to users, while in actuality it would be an executable file that could install a virus onto a system. To prevent this, most programs will display a filename extension by default or allow users to display it if desired.

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