git bundle magic, and other questions

Do there exists some agreed upon common extension for git-bundle, or is 
it just *.bundle (like *.patch)? Do bundles use some magic identifier;
can I add something (and what) to the 'magic' file, so "file <bundle>"
would return "git bundle data" (or equivalent)?

Can I view contents of bundle in gitk or qgit, or giggle?

-- 
Jakub Narebski
Poland