Mysterious OS X File Extension (bug?)

I know that OS X uses the Launch Services.framework to manage the file-extensions. But the icons which belong to the file-extension are part of the Info.plist in the package contents of the 'default-opening-application' of that specific file-extension.

My 'buggy' file-extension is *.cpp used by Xcode as default. Normally it should use the icon given in /applications/xcode/contens/ressources/XXX.icns.

But it doesn't! It's using another cpp icon, which i couldn't find in my whole system - in the Xcode app neither.

After I changed the XXX.icns mentioned above with a custom .icns file and cleared my Icon cache, logged out and restarted my mac:

For roundabout 2 seconds I could see my custom .icns file as the standard icon for the file-extension cpp. This is actually what I want to achieve permanently!

But after a few seconds the icon changes by magic into the other cpp icon, which I can't find on my mac. What is wrong with my Mac.

Little Conclusion:

1. 1 icon that is not to find in any lib or app


2. 1 icon that is default in Xcode for example


3. 1 icon that I want to use for an extension

The (1) icon is shown. But it shouldn't. It should show the (3) if I did everything right, otherwise it needs to show (2).

(1) is the icon shown in the finder (2) is the icon shown in the inspector:

Here is the icons I want to use: