I. Use a program

1. Default Programs Editor

1. Download it from this link.

2. Open the program and select File Type Settings -> Icon.

   

3. Select an extension and press Next.

   

4. Browse for an icon.

   

2. FileTypesMan

1. Download it from this link.
2. Open it, find an extension and Doubleclick it

3. Press the button next to the Default Icon field and select a new icon file.

   

II. Change with the registry

1. Win+R -> “regedit”.

2. Go to the branch:

HKEY_CLASSES_ROOT\[the-extension]\DefaultIcon

If such a branch doesn’t exist check the Data of the (Default) value in this branch

HKEY_CLASSES_ROOT\[the-extension]

This will be the ProgID. Then navigate to the next branch:

HKEY_CLASSES_ROOT\[progid]\DefaultIcon

3. Change/create the String value

(Default)="[path-to-icon],0"

The icon should be of .ico type and the path should have an index in the end.
The index starts from 0 and increases from top to bottom then from the left to the right.

4. Logoff/Restart.

Use Default icons

To select one of the default system icons use “shell32.dll,[id]” as a path. To get the right id use these steps:

1. Go to any folder and open its Properties.

2. Go to Customize tab and press Change Icon.

3. Find an icon and count its index from the top left.
The max index is 305 for Windows 7.