What is a file extension?


File extension is suffix added to the end of filename, which indicates the type of file. Extensions, depending on file system, usually are delimited from the filename with period. Most extensions are three characters long, but it can be from one to more than twenty characters long.

Extensions may be considered as some type of metadata, implying the type of data that is stored in file. Originally, file extension was used to determine file type. The need to condense the file type to three characters often lead to abbreviated/shortened extensions. Examples of that situations are extensions as txt for text, mus for music and others. However, many different softwares started to handle same file types in many different ways, so file extensions started to become closely associated with certain products.

Depending on operating system, filename extensions length can be variable, even some systems alowing multiple extensions on filename. Example, in Unix there are files that contains two extensions, .tar.gz (tar means that file is tar archive of one or more files, gz means that archive is compressed with gzip application.)

Since there are thousands of different file types, there are also thousands of extensions and each can be associated with one or more applications.

Common File Extensions

txt - Plain text file

html - HyperText Markup Language file

ttf - True Type Font file

bak - Backup file

sys - Windows System file

docx - Microsoft Word Document file

wmv - Windows Media Video file

rtf - Rich Text Format file

bin - Binary disk image file

dmg - macOS X disk image file

csv - Comma separated value file

mdb - Microsoft Access database file

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