config.Formatters.JsonFormatter.MediaTypeMappings.Add(
            new QueryStringMapping("type", "json", new MediaTypeHeaderValue("application/json")));

         config.Formatters.XmlFormatter.UseXmlSerializer = true;
         config.Formatters.XmlFormatter.WriterSettings.OmitXmlDeclaration = false;
         config.Formatters.XmlFormatter.WriterSettings.Indent = true;
         config.Formatters.XmlFormatter.MediaTypeMappings.Add(new QueryStringMapping("type", "xml", new MediaTypeHeaderValue("application/xml")));

         services.AddMvc(options =>
         {
            options.FormatterMappings.SetMediaTypeMappingForFormat("xml", "application/xml");
            options.FormatterMappings.SetMediaTypeMappingForFormat("json", "application/json");
            
            options.OutputFormatters.Add(new XmlSerializerOutputFormatter(new XmlWriterSettings()
            {
               OmitXmlDeclaration = false,
               Indent = true
            }));
         }) ;