What Is A File Extension?

Date First Published: 25th September 2023

Topic: Computer Systems

Subtopic: Computer Software

Article Type: Computer Terms & Definitions

Difficulty Level: 4/10

Learn about what a file extension is in this article.

A file extension, also known as a file suffix, or a filename extension, is an ending of a file used to identify the file type. It comes after the full stop in a filename and is usually made up of three to four alphanumeric characters. For example, in a file named textfile.txt, the '.txt' is the file extension, which identifies that the file is a plaintext document. File extensions are used by the operating system to recognise it in order to associate it with the correct program and also provide users with quick insight into the types of files they're working with.

How Do Operating Systems Identify Files By File Extensions?

File extensions can be changed and removed by renaming a file on both Windows and macOS. However, that won't convert the file to another format or change anything about the file other than its identifier, even though the icon might change to the one associated with the new file extension. Programs rely on the file extension for identification and removing or changing it will cause the computer to not know what program to use to view the file. This may cause the file to not open at all or open in gibberish if it succeeds.

macOS and Linux deal with file extensions in a different way to Windows because they don't rely on them to know how to open a file. Linux relies on file extensions when they are available, but it can also use the MIME identifier that is associated with each file. This makes it possible for Linux to open a file in the appropriate application even if the filename lacks an extension. The macOS works in a similar way to Linux, but adds another layer. The additional layer is known as the Uniform Type Identifiers (UTI) framework. The UTI provides a system for uniquely identifying each file type and mapping them to MIME identifiers.

Regardless of how an operating system deals with file extensions, they do nothing more than indicate what the format of a file is supposed to be. An extension does not guarantee the actual format of a file.

Types Of File Extensions

Since there are tens of thousands of different files and file extensions, it is not possible to list them all here. Below are some examples of common file extensions.

• Text and word processing files - doc, docx, odt, pages, rtf, txt, wpd, wps.
• Spreadsheet files - csv, numbers, ods, xls, xlsx.
• Image files - bmp, gif, ico, jpeg, jpg, png, raw, tif, tiff.
• Audio and video files - aif, mov, mp3, mp4, mpg, wav, wma, wmv.
• Presentation software files - key, odp, pps, ppt, pptx.
• Database files - db, .accdb, .nsf, and .fp7.
• Web-related files - asp, aspx, css, htm, html, jsp, php, xml.
• Drawing and graphics program files - afdesign, ai, cad, cdr, drw, dwg, eps, odg, svg, vsdx.
• Email files - email, eml, emlx, msg, oft, ost, pst, vcf.
• Page layout files - afpub, indd, pdf, pdfxml, pmd, pub, qxp.
• Programming files - c, cpp, cs, java, js, json, py, sql, swift, vb.
• System files - bak, cfg, conf, ini, msi, sys, tmpession.
• Archive files - 7z, rar, tar, tar.gz, zip.
• Executable program files. app, bat, bin, cmd, com, exe, vbs, x86.

File Extension Limits

To keep the file name short, most file extensions are no longer than four characters. By default, the file extensions are hidden in all newer versions of Microsoft Windows to make browsing through files more visually appealing, but can be enabled.

As long as the file path, name, and extension do not exceed the maximum file name character limit of the operating system, which is 260 characters for Windows 10, Windows 8, Windows 7, and Windows Vista, 255 characters for Windows XP, and 254 characters for Windows 2000, there is no file extension limit.

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