File extensions – definition and basic information


File extensions are the integral parts of the files that are on each computer. Often, when working on a computer, we encounter a wide range of files, such as docx, ppt, part, mp3 or rmvb. We realize that they are there – but we often do not know what they are for on our computer.

File extension, also known as the filename extension is a suffix added to the name of a computer file, usually consisting of a number of characters placed after the last dot in the filename, which gives us information about its coding. In other words a file extension is a file format. It allows the operating system (e.g. Windows) to recognize what program it should try to open the file.

In the table below you will find a random selection of popular file extensions.


File extension The full name of the file Category

TXT file extension

Plain Text File

Text Files

ICNS file extension

Mac OS X Icon Resource File

System Files

JAVA file extension

Java Source Code File

Developer Files

CPL file extension

Windows Control Panel Item

System Files

CLASS file extension

Java Class File

Developer Files

XCODEPROJ file extension

Xcode Project

Developer Files

CER file extension

Internet Security Certificate

Web Files

RAR file extension

WinRAR Compressed Archive

Compressed Files

DEM file extension

Video Game Demo File

Game Files

7Z file extension

7-Zip Compressed File

Compressed Files

XHTML file extension

Extensible Hypertext Markup Language File

Web Files

IBOOKS file extension

Multi-Touch iBook

Data Files

ASP file extension

Active Server Page

Web Files

DNG file extension

Digital Negative Image File

Camera Raw Files

HQX file extension

BinHex 4.0 Encoded File

Encoded Files