anchor.diff:  
   -spec content_types_provided(#wm_reqdata{}, #context{}) -> {[{string(), atom()}], #wm_reqdata{}, #context{}}.
   content_types_provided(RD, Ctx=#context{}) ->
  -    %% TODO:
  -    %% As I understand S3, the content types provided
  -    %% will either come from the value that was
  -    %% last PUT or, from you adding a
  -    %% `response-content-type` header in the request.
       Method = wrq:method(RD),
       if Method == 'GET' ->
               {[{?XML_TYPE, to_xml}], RD, Ctx};
  
positive.code:                        {'Bucket', [binary_to_list(Bucket)]},
                         {'Key', [Key]},
                         {'UploadId', [binary_to_list(base64url:encode(UploadId))]},
                         {'Initiator',               % TODO: replace with ARN data?
                          [{'ID', [UserKeyId]},
                           {'DisplayName', [UserDisplay]}
                          ]},
  
negative.code:        Method == 'PUT' ->
              {[{?XML_TYPE, doesnt_matter_see_content_types_accepted}], RD, Ctx};
         true ->
              %% this shouldn't ever be called, it's just to
              %% appease webmachine
              {[{"text/plain", unused_callback2}], RD, Ctx}
      end.
  