public async Task PostsToTheCorrectUrl()
            {
                var connection = Substitute.For <IApiConnection>();
                var client     = new FilesClient(connection);

                var newFile = new NewFile(new RawFile("abc.pdf", new byte[] { }, "application/pdf"));
                await client.Create(newFile);

                await connection.Received().Post <File>(Arg.Is <Uri>(u => u.ToString() == "files"),
                                                        Arg.Is <MultipartFormDataContent>(
                                                            nc => nc is MultipartFormDataContent),
                                                        Arg.Is <string>(a => a == "application/json"),
                                                        Arg.Is <string>(c => c == "multipart/form-data"));
            }
            public async Task EnsuresNonNullArguments()
            {
                var client = new FilesClient(Substitute.For <IApiConnection>());

                await Assert.ThrowsAsync <ArgumentNullException>(() => client.Create(null));
            }