Pulp

I'm seeing this in different plugins so filing against core.

Steps to reproduce:
1. Create a file repo and sync it
2. Call http :/pulp/api/v3/content/file/files/?fields=md5

Error:

[04/Oct/2019 20:11:31] "GET /pulp/api/v3/content/file/files/ HTTP/1.0" 200 2234
pulp: django.request:ERROR: Internal Server Error: /pulp/api/v3/content/file/files/
Traceback (most recent call last):
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/viewsets.py", line 114, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/views.py", line 497, in dispatch
    response = self.handle_exception(exc)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/views.py", line 457, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/views.py", line 468, in raise_uncaught_exception
    raise exc
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/views.py", line 494, in dispatch
    response = handler(request, *args, **kwargs)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/mixins.py", line 42, in list
    serializer = self.get_serializer(page, many=True)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/generics.py", line 110, in get_serializer
    return serializer_class(*args, **kwargs)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/serializers.py", line 123, in __new__
    return cls.many_init(*args, **kwargs)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/serializers.py", line 144, in many_init
    child_serializer = cls(*args, **kwargs)
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/pulpcore/plugin/serializers/content.py", line 49, in __init__
    self.fields["artifact"].required = False
  File "/usr/local/lib/pulp/lib64/python3.7/site-packages/rest_framework/utils/serializer_helpers.py", line 148, in __getitem__
    return self.fields[key]
KeyError: 'artifact'
[04/Oct/2019 20:11:46] "GET /pulp/api/v3/content/file/files/?fields=md5 HTTP/1.0" 500 27