MDS does not support the ancillary DAS
Yup, it is missing completely. Fix
For the nc/bears.nc file, which has ancillary DAS attributes in bears.nc.das, those are in the DAS that's is cached. Here's what's in the bears.nc.das ancillary attribute file and what's in the MDS:
So it clearly works, for the netcdf handler. Since the MDS just inserts the DAS it's given, this makes sense. It might be that some of the handlers do not add these attributes correctly, or there is some other problem.
The ancillary attributes appear in the DMR and they work for the FF handler.