On Ubuntu, a directory mimetype is inode/directory
If you run, on Ubuntu:
$ mimetype /home/<your username>
/home/<your username>: inode/directory
There is a “text/directory” in the IANA standards (http://www.rfc-editor.org/rfc/rfc2425.txt and http://www.iana.org/assignments/media-types/text) but it was marked “deprecated”.