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The File Extension Library

What is a file extension?

Short answer: it's the set of letters after the period (.) in the name of a file.

Long answer: File extensions signify the file format of a file. It tells the operating system (OS) how data was stored in the file. Not all OS, require a file extension though. For instance, this is optional in Unix, but is mandatory with MS-DOS and Windows systems.

In most file formats the file type is actually defined by the contents (first bytes, file structure), but extensions are used to define the application that is used for opening the file.

Below is a detailed database of thousands of file extensions our team has compiled for you. Feel free to browse!


Browse all Extensions:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
XLL - Extension Details
Date Added: January 01, 1970
File Extension:  .XLL
Description 1: .XLL is the extension associated with the Excel AddIn File. It is attached to files containing extra templates and functions or other related tools that have to do with enhancing Microsoft Excels capabilities. The .XLL file has plenty in common with a standard .DLL (Dynamic Link Library) file, only it is written specifically for the MS Excel application.
File Type:   Plugin file
Mode: ASCII
Program(s) to Open:  Mac and Windows: Microsoft Excel
Common:
Extension: