const string rootFolder = "C:\src\StartupService\maindirectory";
            var fileSystem = new PhysicalFileSystem(rootFolder);
            var options = new FileServerOptions
            {
                EnableDefaultFiles = true,
                FileSystem = fileSystem
            };

            app.UseFileServer(options);

var physicalFileSystem = new PhysicalFileSystem("C:\src\StartupService\maindirectory");
            var options = new FileServerOptions
            {
                EnableDefaultFiles = true,
                FileSystem = physicalFileSystem,
                StaticFileOptions =
                {
                    FileSystem = physicalFileSystem,
                    ServeUnknownFileTypes = true
                },
                // DefaultFilesOptions = {DefaultFileNames = new[] {"index.html"}}
            };
            app.UseFileServer(options);