OK Understood.
Best tool for doing that IMO is
WAssociate, a context menu management utility.
Once you have installed it, you can create a shortcut to it in the sendto folder (I am not sure it does it itself) and after that you can just right-click on any file and send it to that application.
You'll see there is an icon button on it's GUI, click on it and browse to the new icon of your choice for the filetype you edit.
After doing that you may need to refresh the icons with an extra tool to see immediately the change in explorer.
That's the easier/fastest way to do it.
But you can also go in Control Panel / Folder Options / File Types in Windows and select an extension, click on the Advanced tab and change the icon. (That's on WinME, it may differ a bit on the OS you have)
Does this answer your question ?