File extensions: what they are, how many exist and who invented them

The file extensions are strings of characters, usually consisting of three or more letters after the period in a file name (for example .txt, .jpg, .docx and so on). They are useful for identifying the file type and the program that can be used to verify and manage its contents.

Operating systems such as Windows, macOS, Linux but also, on the mobile side, Android and iOS, make extensive use of file extensions to automatically associate files with specific programs based on their type. A file with the .docx extension is associated with a word processing program such as Microsoft Word or other software capable of reading the same document format.

Who invented file extensions

The idea of ​​file extensions can be traced back to Gary Kildallfounder of Digital Research and American computer scientist who developed the CP/M operating system in the 1970s. CP/M was one of the first microcomputer operating systems, and file extensions were used to distinguish various file types. THE operating systems released later adopted the same choice and the practice began to spread on a large scale.

The first demonstrations of the use of file extensions date back to 1961 with the WITH Compatible Time-Sharing System (CTSS), a pioneering operating system that associated a label or a suffix to a file to identify its type and format. CTSS used a file name system that included a one or two letter “keyword” at the end of each name.

The use of file extensions became a common standard in operating systems starting in the late 1970s and throughout the 1980s: PC-DOS, MS-DOS, Macintosh, UNIX, and Linux are some examples. Although Macs are known to use so-called metadata to identify the type of file, extensions are however widely used. UNIX did not initially use file extensions but their use nevertheless became widespread and was later confirmed also on Linux.

What were the rules for naming files in MS-DOS first and then in Windows

Initially, MS-DOS used an i file names known as File Allocation Table 8.3 (FAT 8.3). This naming standard was based on a few key rules: file names could have a maximum of 8 characters in the main name and 3 characters in the extension. Furthermore, names could only include alphanumeric characters (AZ, 0-9) and some well-defined special characters such as the underscore “_” and the period “.”

With the advent of Windows, especially with the introduction of Windows 95 and later, changes have been made to the file name structure. To support the long file namesVFAT (Virtual FAT) changed the way the file system organized the contents of a directory. So Microsoft activated the possibility of using descriptive names up to 255 characters long, opening up the use of spaces and a wider range of special characters.

More advanced file systems such as NTFS have subsequently introduced additional features but some remain limitations: File names cannot contain the characters: \ / : * ? " < > | and must not end with a period or a space. Some restrictions concern the names that can be assigned to files and folders which, in today’s Windows operating systems, take us back to the 1980s.

Although Windows also allows you to use uppercase and lowercase letters in file names, the operating system remainsnon case-sensitive“: this means that at low level there is no difference between files written with uppercase or lowercase characters (unlike Linux and systems Unix-like).

How many file extensions exist

There are thousands of different file extensions representing a wide range of file types and formats. We said that the extension helps to understand the contents of each file.

By default, unfortunately, Windows still abstains from the View file extensions for known file types. In other words, Windows already knows that a file .docx can be opened with Word or another word processor so it does not show the corresponding extension.

The advice is to press Windows+E to open File Explorer, then click the tab View and finally tick the box File name extensions. Viewing extensions in Windows also helps to “unmask” the tricks put in place by those who want to pass off a file as something it isn’t, for example by using a double extension or doing “magic tricks” with the character set used.

File extensions they are also used to identify a type of file distributed online: modules URL rewriting they allow you to customize the addresses of the pages served by each Web server, but basically the files are for example .htm, .html, .php, .rb, .jsp, .cgi on the server side. Checking the source of any page Web (CTRL+U in any browser) you come across CSS style sheets (.css), JavaScript scripts (.js), images and multimedia files, character fonts, JSON data files, XML, CSV,… All this to say that extensions files are practically everywhere.

How to check the type of a file

As a general rule, the extension assigned to a file helps you understand what type of content it hosts. It cannot and must not be taken at face value. A file can be assigned a different extension than its content.

In case of a missing file extension or a file that does not want to open with the program that should read it, we suggest – as explained in the article – to use software like TrIDNet which recognizes any object by examining its headers. Before saying that a file is damaged, it is a good idea to inspect it header (headers) or the initial part of a file that contains metadata, formatting information and data that identify the content and characteristics of the file itself.

Header file: what is it

When we talked about binary code, we also explained what a hex editor: a program like HxD allows you to open any file by checking its header. The presence of strings such as JPG, PNG, TIFF, PDF, [Content_Types].xml constitutes a valuable suggestion on the real typology of the file.

The reference [Content_Types].xmlfor example, is often associated with documents created in the file format Microsoft Office Open XML (for example .docx, .xlsx, .pptx).

Change a program’s assignment to a file extension in Windows

What is always good to keep in mind is that Windows does not check the file headers but simply checks the extension associated with each element.

Typing Choose a default app for each file type in your operating system’s search box, you can change the default program to open each file. The approach is substantially identical in Windows 10 and Windows 11. You can search for the extension you are interested in and then specify the files that, by default, must be opened with a specific application.

Programs associated with Windows file extensions

In case of problems, it is always possible reset file associations in Windows by restoring the default settings. To do this, you can type Predefined apps in the Windows search box, scroll to the bottom of the window and then click Restore o Reset next to or below the entries Restore all default apps (Windows 11) o Restore the default settings recommended by Microsoft (Windows 10).

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