@Override
    public CompletableFuture<ResponseHeaders> readHeaders(Executor fileReadExecutor) {
        return readAttributes(fileReadExecutor).thenApply(this::readHeaders);
    }
Generates an entity tag of the file with the given attributes using the {@code entityTagFunction}
which was specified with the constructor.

@return the entity tag or {@code null} if {@code entityTagFunction} is {@code null}.
core/src/main/java/com/linecorp/armeria/server/file/AbstractHttpFile.java