A concern adapted from the GoRails ActionMailbox episode https://gorails.com/episodes/save-action-mailbox-html-emails-with-action-text?autoplay=1

mail_params.rb

# mailboxes/concerns/mail_params.rb
module MailParams
  extend ActiveSupport::Concern

  def mail_params
    attachments = build_attachments
    body = build_rich_body(attachments)
    { subject: mail.subject, body: body, attachments: attachments.map { |a| a[:blob] } }
  end

  def build_attachments
    mail.attachments.map do |attachment|
      blob = ActiveStorage::Blob.create_after_upload!(
        io: StringIO.new(attachment.decoded),
        filename: attachment.filename,
        content_type: attachment.content_type,
      )
      { original: attachment, blob: blob }
    end
  end

  def build_rich_body(attachments)
    if mail.multipart? && mail.html_part
      document = Nokogiri::HTML(mail.html_part.body.decoded)

      attachments.map do |attachment_hash|
        attachment = attachment_hash[:original]
        blob = attachment_hash[:blob]

        if attachment.content_id.present?
          # Remove the beginning and end < >
          content_id = attachment.content_id[1...-1]
          element = document.at_css "img[src='cid:#{content_id}']"

          element.replace "<action-text-attachment sgid=\"#{blob.attachable_sgid}\" content-type=\"#{attachment.content_type}\" filename=\"#{attachment.filename}\"></action-text-attachment>"
        end
      end

      document.at_css("body").inner_html.encode("utf-8")
    elsif mail.multipart? && mail.text_part
      mail.text_part.body.decoded
    else
      mail.decoded
    end
  end
end

# models/post.rb
class Post < ApplicationRecord
  has_rich_text :body
  has_many_attached :attachments
end

# mailboxes/posts_mailbox.rb
class PostsMailbox < ApplicationMailbox
  include MailParams

  def process
    Post.create mail_params
  end
end

The original PostsMailbox is here: https://github.com/gorails-screencasts/action-mailbox-action-text/blob/master/app/mailboxes/posts_mailbox.rb

Thank you for the gist !
FYI, I tested with a gmail inbound email, I got an error line 33 : element can be nil and element.replace fails.
You can add a next unless element line 34 to fix it.

@jumichot Interesting, but then your text content won't feature the attachment. We could simply append the action-text-attachment. But maybe that content_id appears inside another HTML tag?
What does the mail.html_part.body.decoded look like?

My test case : 2 attachments

• one embedded direclty in the email body (via drag and dropping an image in gmail interface), the attachment is present in the html via an image tag
• and one another regular attachment that doesn't appear in the html body (that cause the problem)

Both attachments have content_ids

The body looks like that :

<div dir=3D"ltr"><div><div dir=3D"ltr" class=3D"gmail_signature" data-smart=
mail=3D"gmail_signature"><div dir=3D"ltr"><div><div dir=3D"ltr"><div dir=3D=
"ltr"><div><br></div></div></div></div></div></div></div><div class=3D"gmai=
l_quote"><br><div dir=3D"ltr">Bonjour Julien,<div><br></div><div>Ceci est u=
n test, pouvez-vous intervenir au plus vite =C3=A0 l&#39;adresse indiqu=C3=
=A9e ci dessous ?</div><div><br></div><div>Merci d&#39;avance<img src=3D"ci=
d:ii_kncy5jja0" alt=3D"Screenshot 2021-04-10 at 10.33.18.png" width=3D"543"=
 height=3D"126" style=3D"margin-right:0px"><br><div><div dir=3D"ltr" data-s=
martmail=3D"gmail_signature"><div dir=3D"ltr"><div><div dir=3D"ltr"><div di=
r=3D"ltr"><div>Cordialement,=C2=A0</div><div><br></div><div>--</div><div><b=
>Julien</b></div><div><br></div><div><br></div><br></div></div></div=
></div></div></div></div></div>
</div></div>

and my .mail.attachments.map(&:content_id) looks like that ["<ii_kncy5jja0>", "<f_kncy6vaw1>"]

The attachment with the content id f_kncy6vaw1 is not present in the HTML and raise an error.

Alright, then I guess we need to append the attachment.
Also, we may want to delete the signature.
I'll revisit that sometime later, this feature is on pause for a couple of days.