Well, after some time trying different approaches I finally managed to make it work with:

      %HTTPoison.AsyncStatus{code: code} when code >= 400 ->
        # So, turns out HTTPoison have the very convenient type AsyncStatus!
        {:stop, code, state}

      %HTTPoison.AsyncChunk{chunk: chunk} ->
        IO.binwrite(file, chunk)
        {:noreply, state}

      %HTTPoison.AsyncHeaders{headers: headers} ->
        headers_map = Enum.into(headers, %{})
        # Now I am able to access all headers just like a regular map!
        IO.inspect(headers_map["Content-Length"], label: "Content length: ")
        {:noreply, state}

      %HTTPoison.AsyncEnd{} ->
        File.close(file)
        {:stop, :normal, state}

      %HTTPoison.Error{reason: reason} ->
        {:stop, reason, state}

      _ ->
        {:noreply, state}

Welcome to the forum! I’m glad you were able to get something working.

What was the format of headers before the Enum.into call? If it was a Keyword List then you could use Keyword.get(headers, "Content-Length") to get the Content-Length header. A keyword list is a better format to work with for headers since headers can be repeated with different values and you can’t represent that with a map since a map cannot have duplicate keys.

It’s a proplist. There’s an erlang module to manage them. For example:
:proplists.get_value("abc", [{"abc", 123}])