Why does a file have a extension of 3 or 4 letters following a period?

A file extension of 3 or 4 letters following a period indicates the file type and format, allowing the operating system to recognize how to open and handle the file. For example, ".txt" signifies a plain text file, while ".jpg" indicates an image file. This naming convention helps users and software quickly identify the file's content and associated applications. Extensions also facilitate compatibility across different platforms and programs.