If you query /pulp/api/v2/repositories/my_repo/search/units/ with

{'criteria': {'fields': {'unit': ['filename']}, 'type_ids': ['rpm'], 'filters': {}}}

you will get results like:

...
  u'metadata': {u'_id': u'eb6fa0ea-0bff-4fe6-854b-68364f1bae00',
                u'filename': u'abrt-addon-kerneloops-2.1.11-2.el7.x86_64.rpm'},
...

If you omit the type_ids:

{'criteria': {'fields': {'unit': ['filename']}, 'filters': {}}}

you will get results like:

...
   u'metadata': {u'_content_type_id': u'rpm',
                u'_id': u'eb6fa0ea-0bff-4fe6-854b-68364f1bae00',
                u'_last_updated': 1390620260.0,
                u'_ns': u'units_rpm',
                ...

i.e. the fields part of the criteria is ignored and all fields are returned.  I believe that this is a bug.  If it's intentional, please update the documentation.  Thanks.