Some examples of file extensions include. For the most part, file extensions consist of three characters, which are typically letters or digits, that textually represent the type of file it is. There are no particular rules regarding how an extension should be formatted other than it must begin with a period and have at least one character after it. This extension allows the operating system to know what type of file it is and what program to run when you double-click on it. For example, the file extension.txt has an extension of.
