If I understand correctly, you tried to add a record with registry import, right ?
warning, registry import is not additive, when you have multiple values like
text/html
text/x-web-textile
if you import another value, the prior one will be deleted.
and the imported values can be controlled by the registry, here from
<value_type type="plone.registry.field.Choice">
plone.app.vocabularies.AllowableContentTypes
also you should notice from this thread that
content_type_registry/manage_predicates is the way to go.