On the Extensions page, you can specify relations between file extensions and your program(s).

Extension

Specify the extension to be registered.

Examples:
aip
jpg

Associated File

Specify the program to be used for executing files with this extension.

Description

In the Description field, you can add a description for the extension.

Icon File

This setting specifies the icon to associate with documents with the given Extension. This file can be an icon file, an executable containing icons (.exe) or a DLL containing icons.

Icon Index

If the Icon file contains multiple icon resources, you can use this setting to specify the index of the icon resource to use. Normally, you want to leave this as zero.