What is a File Extension and What is it For

In computing, a file extension is a suffix that is appended to a file name to indicate how the data is organized within the file. The software application linked to a particular file type must be able to access the file’s contents in the right format.

File extensions, usually composed of three or four alphanumeric characters, appear after the period in a filename. They give users a quick overview of the different kinds of files they are working with. An operating system may use file metadata or rely only on the file extension to decide which application to launch.

MIME (Multipurpose Internet Mail Extensions) offers a mechanism for recognizing various file formats, enabling file sharing and system-to-system opening across the internet. Linux can open a file in the correct application even if the filename doesn’t have an extension, thanks to the MIME identifiers. The Linux operating system adds the Uniform Type Identifiers (UTI) framework as an additional layer to the macOS operating system.

File extensions themselves do nothing more than specify the intended format of a file, regardless of how an operating system handles them. With the vast number of file extensions in the world of computers, it is impractical to list them all in one article.

The Meaning of File Extensions

There are many distinct file types on your computer, and each one has a unique file extension. A file extension is a three- or four-letter code that appears after the period and at the end of a file name. These extensions provide information about a file’s properties and intended use.

Here are some examples of file extensions:

  • The .JPG or .JPEG extension (such as IMAGE.JPG) is used by JPEG image files.
  • The .DOCX extension is used for Word documents, while older versions may use .DOC (e.g., COVERLETTER.DOCX).
  • The .MP3 extension is used by MP3 audio files, such as RHYME_RAP.MP3.

File Extensions for Folders

The type ("file" versus "directory") of a directory is maintained in the master file table instead of having a defined extension in Windows. Folders, also known as directories, are actually files with a FileAttribute value designating them as folders rather than ordinary files.

Folders are a distinct class of object and do not require an extension to be recognized as folders. Unlike files, folders are not objects with a real size. Instead, the size of all the contents assigned to a folder is calculated. While both files and folders are listed in the file tables, you can differentiate between the two using certain attributes, but an extension is not one of them.