static HttpFile from(CompletionStage<? extends HttpFile> stage) {
        return new DeferredHttpFile(requireNonNull(stage, "stage"));
    }
Returns an {@link HttpFile} that becomes readable when the specified {@link CompletionStage} is complete.
All {@link HttpFile} operations will wait until the specified {@link CompletionStage} is completed.
core/src/main/java/com/linecorp/armeria/server/file/HttpFile.java