@Streaming(true)
        @Get("/fileFuture")
        public CompletableFuture<HttpFile> fileFuture() {
            return UnmodifiableFuture.completedFuture(HttpFile.of(HttpData.ofUtf8("fileFuture")));
        }
A mock service for testing {@link HttpFileResponseConverterFunction}.
core/src/test/java/com/linecorp/armeria/internal/server/annotation/ResponseStreamingTest.java