See https://hacks.mozilla.org/2013/01/introducing-web-activities/ for
more information.

Unfortunately, whole activities are build around MIME types, and I am
not sure we will have a correct one set anyway.

According to
http://www.openldap.org/lists/openldap-general/200001/msg00028.html the
best available mime type is probably

    text/directory

defined in ftp://ftp.isi.edu/in-notes/rfc2425.txt