Problem description
API Design Guidelines requirement is that datacontenttype field is media-type that describes the event payload encoding, must be application/json for CAMARA APIs
In https://github.com/camaraproject/Commonalities/blob/main/artifacts/notification-as-cloud-event.yaml it is defined as follows:

datacontenttype:
          type: string
          description: 'media-type that describes the event payload encoding, must be "application/json" for CAMARA APIs'

Possible evolution
Define required value as enum - as the only possible value is "application/json"

Alternative solution

Additional context
The similar specversion field is defined as enum with PR:

• guideline_for_specversion_cloudevent_concept_as_enum #187

The datacontenttype field is optional , specversion field is mandatory