fix: Save content_type, multipart information to email message model (#2774)

spec/mailboxes/support_mailbox_spec.rb

@@ -8,7 +8,10 @@ RSpec.describe SupportMailbox, type: :mailbox do
     let!(:channel_email) { create(:channel_email, account: account) }
     let(:support_mail) { create_inbound_email_from_fixture('support.eml') }
     let(:described_subject) { described_class.receive support_mail }
-    let(:serialized_attributes) { %w[text_content html_content number_of_attachments subject date to from in_reply_to cc bcc message_id] }
+    let(:serialized_attributes) do
+      %w[bcc cc content_type date from html_content in_reply_to message_id multipart number_of_attachments subject
+         text_content to]
+    end
     let(:conversation) { Conversation.where(inbox_id: channel_email.inbox).last }
 
     before do