rails: ActionMailer fails to create correct multipart message with rfc822 attachment

Steps to reproduce

# frozen_string_literal: true

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'
  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
  gem 'rails', github: 'rails/rails', branch: 'main'
end

require 'minitest/autorun'
require 'action_mailer/railtie'

class TestMailer < ActionMailer::Base
  def main
    message_to_attach =  <<~MAIL.gsub(/\n/, "\r\n")
      Message-ID: <12345@foo.example>
      Subject: Test
      From: test1@foo.example
      To: test2@foo.example
      
      hello world
    MAIL
    attachments['test.eml'] = {
      mime_type: params[:mime_type],
      content: message_to_attach
    }

    mail(from: 'test3@foo.example', to: 'test4@foo.example', subject: 'Test outer') do |format|
      format.text { "outer text - #{params[:mime_type]} attachment" }
    end
  end
end

class BugTest < Minitest::Test
  def test_with_plain_mime_type
    message = TestMailer.with(mime_type: 'text/plain').main
    assert_equal message.mime_type, 'multipart/mixed'
  end

  def test_with_rfc822_mime_type
    message = TestMailer.with(mime_type: 'message/rfc822').main
    assert_equal message.mime_type, 'multipart/mixed'
  end
end

The second test fails, the message object is rendered as follows:

Date: Fri, 23 Jul 2021 13:59:19 +0200
From: test3@foo.example
To: test4@foo.example
Message-ID: <60faaf1738a02_14fbd98-3fb@flo.adigi.ai.mail>
Subject: Test outer
Mime-Version: 1.0
Content-Type: text/plain;
 boundary="--==_mimepart_60faaf1738008_14fbd98-484";
 charset=UTF-8
Content-Transfer-Encoding: 7bit


----==_mimepart_60faaf1738008_14fbd98-484
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit

outer text - message/rfc822 attachment
----==_mimepart_60faaf1738008_14fbd98-484
Content-Type: message/rfc822
Content-Transfer-Encoding: 
Content-Disposition: attachment;
 filename=test.eml
Content-ID: <60faaf1739102_14fbd98-2b@flo.adigi.ai.mail>

Message-ID: <12345@foo.example>
Subject: Test
From: test1@foo.example
To: test2@foo.example

hello world

----==_mimepart_60faaf1738008_14fbd98-484--

Expected behavior

The message.mime_type is multipart/mixed in both test cases. ActionMailer handles attachments in the way it is described in https://guides.rubyonrails.org/action_mailer_basics.html#adding-attachments describes.

Actual behavior

When attaching a message/rfc822 - an email message - the generated mail parts are not nested correctly, the outer multipart/mixed is missing.

Perhaps this behaviour is caused by the special handling of such attachments in the mail gem.

System configuration

Rails version: master

Ruby version: 3.0.2