One thing you can do, if you are using TAureliusDataset, is create a transient property in your entity class like this:
property RawContent: TArray<Byte> read GetRawContent;
and then implement it like this:
function TMyEntity.GetRawContent: TArray<Byte>;
begin
Result := DecodeBase64(YourBase64PropertyContent);
end;
Then with TAureliusDataset you can add a field RawContent
, and that field can be read by Report Builder as raw binary value.