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.