带有 ical 附件的多部分电子邮件显示不正确

问：

我不确定这是否是我的代码、ActionMailer、Mail 甚至 icalendar gem 的问题？

用户注册一个活动，他们会收到一封带有 ical 附件的电子邮件：

# app/mailers/registration_mailer.rb

class RegistrationMailer < ApplicationMailer
  helper MailerHelper

  def created(registration)
    ...

    cal = Icalendar::Calendar.new
    cal.event do |e|
      e.dtstart = @event.start_time
      e.dtend = @event.end_time
      e.organizer = 'mailto:[email protected]'
      e.attendee = @recipient
      e.location = @location.addr_one_liner
      e.summary = @summary
      e.description = @description
    end
    cal.append_custom_property('METHOD', 'REQUEST')
    mail.attachments[@attachment_title] = { mime_type: 'text/calendar', content: cal.to_ical }

    mail(to: @recipient.email, subject: "[20 Liters] You registered for a filter build on #{@event.mailer_time}")
  end

  ...
end

我有文本和 HTML 视图：

• app/views/registration_mailer/created.text.erb
• app/views/registration_mailer/created.html.erb

当我省略附件时，电子邮件的结构如下：

Header stuff...
Mime-Version: 1.0
Content-Type: multipart/alternative; boundary="--==_mimepart_63358693571_1146901122e"; charset=UTF-8
Content-Transfer-Encoding: 7bit

----==_mimepart_63358693571_1146901122e
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

[the text version of the email here]

----==_mimepart_63358693571_1146901122e
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit

[the HTML version of the email here]
----==_mimepart_63358693571_1146901122e--

当附件存在时，电子邮件的结构如下：

Header stuff...
Mime-Version: 1.0
Content-Type: multipart/mixed; boundary="--==_mimepart_6335c3388b140_114924286ed"; charset=UTF-8
Content-Transfer-Encoding: 7bit

----==_mimepart_6335c3388b140_114924286ed
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

[the text version of the email here]

----==_mimepart_6335c3388b140_114924286ed
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit

[the HTML version of the email here]

----==_mimepart_6335c3388b140_114924286ed
Content-Type: multipart/alternative; boundary="--==_mimepart_6335c3389bc30_114924287a3"; charset=UTF-8
Content-Transfer-Encoding: 7bit

----==_mimepart_6335c3389bc30_114924287a3
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

[the text version of the email AGAIN]

----==_mimepart_6335c3389bc30_114924287a3
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit

[the HTML version of the email AGAIN]

----==_mimepart_6335c3389bc30_114924287a3--
----==_mimepart_6335c3388b140_114924286ed
Content-Type: text/calendar; charset=UTF-8
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=20Liters_filterbuild_20221011T0900.ical
Content-ID: <[email protected]>

[numbers and letters]
----==_mimepart_6335c3388b140_114924286ed--

突然变成了一棵奇怪的树：

1. Content-Type: multipart/mixed
    A. Content-Type: text/plain
    B. Content-Type: text/html
    C. Content-Type: multipart/alternative
        i. Content-Type: text/plain
        ii. Content-Type: text/html
    D. Content-Type: text/calendar

Rails 的邮件预览不会重现这个问题，也没有使用 Litmus 的电子邮件客户端预览（因为它似乎删除了文本部分和附件），但我假设内容类型的变形结构这不仅仅是特定于客户端的渲染问题。

我认为这来自 ActionMailer 下面的 Mail gem，它奇怪地构建了内容类型，但我在这里有点超出了我的深度。可能是 ActionMailer，我真的不知道怎么说。

我不是很精通这个，但我想我想要这个结构：

1. Content-Type: multipart/mixed
    A. Content-Type: multipart/alternative
        i. Content-Type: text/plain
        ii. Content-Type: text/html
    B. Content-Type: text/calendar

所以，两个问题：

1. 如果这是我的代码，我做错了什么？

2. 如果它不是我的代码，我可以强制使用我想要的结构吗？

我一直在梳理 ActionMailer 和 Mail 代码库，但还没有找到一种方法来手动将我的电子邮件形成到这个级别。