Incorporate ExternalFile in CustomDatum?

Extracted from !23+

The ExternalFile could be incorporated as CustomDatum, too:

export interface ExternalFile {
  description?: string;
  fileReferenceURI?: string;
  fileType?: string;
}

// RfC
export interface ExternalFile extends CustomDatum {
  comment?: string;
  referenceURI?: string;
  type: "ExternalFile";
}