public String getCachedContentId() {
    return cachedContentId;
  }
This is a wrapper around a {@link DtpDigitalDocument} that encodes a blobby file.
Data in the {@link DtpDigitalDocument} has a 1:1 relationship with the Schema.org spec, fields
this class represent DTP specific data that doesn't fit into the schema.org representation.
portability-types-common/src/main/java/org/datatransferproject/types/common/models/blob/DigitalDocumentWrapper.java