Mac – Set macvim as default application after homebrew installation

I've installed macvim via homebrew. This is working fine.

But when it commes to choose a default application for a new file type I cannot see MacVim. How can I set MacVim as the default application for a file type when it has been installed with homebrew?

Note: The question is not about how to select a default application in Finder, the question is how to set an application as default which is not listed in /Applications and there fore not shown by the Finder as a possible default application.