How can I make sure that `FileIO.fromPath` is picking up my dispatcher?

I’m trying to change the underlying dispatcher of FileIO. The docs say that it is using akka.stream. blocking-io-dispatcher. So I configured it like this:

akka.stream {
  blocking-io-dispatcher = "my-dispatcher"
  }
}

my-dispatcher {
  type = Dispatcher
  executor = "thread-pool-executor"
  thread-pool-executor {
    fixed-pool-size = 10
  }
  throughput = 1
}

But just to be sure, I also set it explicitly in code

  FileIO
    .fromPath(source.value)
    .withAttributes(ActorAttributes.dispatcher("my-dispatcher"))
    .log(s"reading ${source.value}")
    .fold(ByteString.empty)(_ ++ _)
    .map(_.toArray)
    .map(ImmutableImage.loader().fromBytes)
}

But the log statements are still being written using the default dispatcher. Why?

12:01:47.624UTC DEBUG[default-akka.actor.default-dispatcher-4] [akka.stream.Log(akka://default/system/Materializers/StreamSupervisor-0)] - [reading /Users/oliver/Downloads/e7lp1y1tuwt41.jpg] Element: [...]

Is the underlying file operation still done on my thread pool? Is logging just working on another thread pool? Something wrong with my assumptions or configuration?

How can I prove that the reading is done by my own thread pool?

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