A file extension or file name extension is the ending of a file that helps identify the type of file in operating systems such as Microsoft Windows and any other operating systems that you might be using.
On the Windows Operating System, the file name extension is a period that is often followed by three characters, but may also be one, two, or four characters long.
By TechnoMag Reporter
A file extension, sometimes called a file suffix or a filename extension, is the character or group of characters after the period that make up an entire file name.
For example, the file ends in docx, a file extension that might be associated with Microsoft Word on your computer.
Any file’s extensions can be renamed but that won’t convert the file to another format or change anything about the file other than this portion of its name.
Filename extensions may be considered a type of metadata. They are commonly used to imply information about the way data might be stored in the file.
The exact definition, giving the criteria for deciding what part of the file name is its extension, belongs to the rules of the specific filesystem used; usually the extension is the substring which follows the last occurrence, if any, of the dot character (example:
txt is the extension of the filename
html the extension of
Phones running on Windows Mobile use XAP files while Android have apk files.