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).
This make them recognized by file managers and stuff. Maybe one day we
should properly register mime types in the "vnd." namespace with IANA,
but I am too lazy to deal with the bureaucracy for that, hence let's
stick with the x. namespace for now.
This defines confext/sysext DDIs as subtype of:
https://www.iana.org/assignments/media-types/application/vnd.efi.img
Which is what everyone appears to use for raw disk images, in particular
if they contain a GPT partition table.
