During initial work on DM-17538, I encountered a bug that both master biases and master flats are created with FILTER = _unknown_ 

This creates a problem when the calibration products are ingested, because the registry assigns them that value instead of the correct filter, and they cannot be used properly for image processing. (Manually editing the registry is the present workaround.)

The bug exists for biases, flats, and also sky frames (per Shenming Fu's report on Slack in #dm-decam on 17 April 2020).

When this is resolved, the if/else block in obs_decam's ingestCalibs translate_filter method should be removed, per the comment there.