added icon to OSX app, which is also registered for .ipynb files

The extension could be added in fact, but it's not necessary. I knew that it was working on my system, but I now checked that Apple states it "officially":
> In OS X, the CFBundleTypeIconFile key contains the name of an icon resource file with the document icon. An icon resource file contains multiple images, each representing the same document icon at different resolutions. If you omit the filename extension, the system looks for your file with the extension .icns.

this is taken from https://developer.apple.com/library/mac/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html#//apple_ref/doc/uid/TP40009249-SW9