The range of dcat:mediaType has been tightened from dct:MediaTypeOrExtent to dct:MediaType as part of the revision of DCAT. https://www.w3.org/TR/vocab-dcat-2/#Property:distribution_media_type

Currently a URI or a literal is returned. https://github.com/ckan/ckanext-dcat/blob/master/ckanext/dcat/profiles.py#L1411 Only a URI should be used at this point.