oro_api:
    entities:
        Oro\Bundle\AttachmentBundle\Entity\Attachment: ~
        Oro\Bundle\AttachmentBundle\Entity\File:
            fields:
                content:
                    data_type: string
                    depends_on: [filename]

oro_api:
    entities:
        Oro\Bundle\AttachmentBundle\Entity\Attachment: ~
        Oro\Bundle\AttachmentBundle\Entity\File:
            fields:
                content:
                    data_type: string
                    depends_on: [filename]
            actions:
                create: false
                update: false
                delete: false
                delete_list: false
                get_subresource: false
                get_relationship: false
                add_relationship: false
                update_relationship: false
                delete_relationship: false

Create customize_loaded_data processor to fill Content field for File entity. Make File entity read-only because this entity can be used only as a part of other entity
BAP-11049: Create customize_loaded_data processor to fill Content field for File entity. Make File entity read-only because this entity can be used only as a part of other entity
