Basic information about computer files and file extensions
The idea of file extensions is pretty old and file suffixes are still used by many current computer operating systems (Microsoft Windows 7, Windows Vista, Windows XP, Windows 95/98/ME/2000 or Windows Mobile, Apple MAC OS X, Linux/Unix etc.).
A file extension basically tells the OS what kind of data file it is, how it can work with it and especially what program or application the file extension is associated with, ie. what program can open, view, play, edit, convert, burn, print or do some other specific action with that file.
Basic structure of computer filename consists of base filename and file extension (also called filename suffix). The file extension is separated from base name by a period - ie. filename.extension .
The base name of the file may contain multiple periods, but only the last one is the divider between the name and actual file extension.
File name can use any characters, numbers and some symbols, but cannot contain some special characters reserved by the operating system (the illegal characters for Windows are \ / * : < > ? |).
What is a file extension?
Generally computer files are named with a three or four character file extension to denote file type (filename.doc, filename.xls, filename.txt, videofilename.avi, audiofilename.mp3) and associate the file to a software application that is able to open it, ie. Microsoft Word opens the .doc files, Microsoft Excel opens the .xls files, Notepad opens the .txt files, Windows Media Player plays the .avi files, Winamp plays .mp3 files etc..
In modern computer operating systems, like Microsoft Windows, files can have extensions with more than three characters and the length of the file extension is not limited. Four character suffixes are commonly used (e.g. HTML (HTML page), FLAC (Flac audio file), XLSX (Excel spreadsheet), DOCX (Word document) and others).
However most file extensions still uses only three characters, mainly because of tradition, but sometimes also because of backward compatibility reasons, such as compatibility for MS-DOS based systems and its programs.
In some operating systems (like Unix) is the file extension purely optional, while in some others (DOS or Windows) it is a requirement. Some older operating systems limit the length of the extension (DOS and OS/2 to three characters) while other (Microsoft Windows, Unix or Apple OS X) do not limit it at all. Some operating systems do not use filename extensions at all and use another way of recognizing various file types through additional file metadata, or binary recognition. Unix accepts the separator dot as a legal character, but does not give it a special recognition on the OS level.
Here at File-extensions.org you can find a huge searchable file extensions library that contains thousands of file extension records. A large number of file type entries have detailed explanation of each file type and the way they are used today and also contain linked programs that can view, open, edit, convert or play unknown file type you are looking for.