mime: expose a mime type for encrypted credentials

mime: expose a mime type for encrypted credentials

Let's make things nice for desktops, and provide a mime type for
credential files.

This uses the 128bit header identifier that our credential files start
with. However, the files are always base64 encoded, hence we have to
match the base64 string, hence add a small test case that generates them
properly for us, and truncates them at the right place (since 128 is not
evently divisable by 6).