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What are File Extensions?

File system is an important functional component of an operating system. Any information which needs to be stored on the computer is stored within the file system. To distinguish between files with different types of content file extensions are used.

File extension is a short string which is attached to the end of the filename with a dot in front of it. In many cases file extension completely defines in what format information is stored inside the file. For example all files with PNG extension will contain Portable Network Graphics and all files with ZIP file extension will contain archive in ZIP format. But at the same time certain file extension just define a purpose of the file but not its content. For example BAK file typically defines that file contains some kind of backup but it is impossible to know which application can process BAK file unless you know the file origin or look inside the file.

File extension provide an easy mechanism to know what is inside the file before looking inside it. In many cases operating system will be able to choose a corresponding application to open a file just by checking a file extension. Same applies when operating system needs to search for a files of a specific format. For example search for images will just need to look for all files with extensions matching image files. Without file extensions operating system would have to look into contents of each individual file to find matching files.

To know which file extension matches which application, operating system creates a database with matches. When user tries to open a file operating system will look into matches database to find which application can open it. If there are multiple applications matching the extension then default one will be selected.