FIX: Notification emails with attachments are incorrectly structured

Two behaviors in the mail gem collide:

 1. Attachments are added as extra parts at the top level,
 2. When there are both text and html parts, the content type is set to
    'multipart/alternative'.

Since attachments aren't alternative renderings, for emails that contain
attachments and both html and text parts, some coercing is necessary.

lib/email/sender.rb

@@ -270,6 +270,50 @@ module Email
           )
         end
       end
+
+      fix_parts_after_attachments!
+    end
+
+    #
+    # Two behaviors in the mail gem collide:
+    #
+    #  1. Attachments are added as extra parts at the top level,
+    #  2. When there are both text and html parts, the content type is set
+    #     to 'multipart/alternative'.
+    #
+    # Since attachments aren't alternative renderings, for emails that contain
+    # attachments and both html and text parts, some coercing is necessary.
+    #
+    # When there are alternative rendering and attachments, this method causes
+    # the top level to be 'multipart/mixed' and puts the html and text parts
+    # into a nested 'multipart/alternative' part.
+    #
+    # Due to mail gem magic, @message.text_part and @message.html_part still
+    # refer to the same objects.
+    #
+    def fix_parts_after_attachments!
+      has_attachments = @message.attachments.present?
+      has_alternative_renderings =
+        @message.html_part.present? && @message.text_part.present?
+
+      if has_attachments && has_alternative_renderings
+        @message.content_type = "multipart/mixed"
+
+        html_part = @message.html_part
+        @message.html_part = nil
+
+        text_part = @message.text_part
+        @message.text_part = nil
+
+        content = Mail::Part.new do
+          content_type "multipart/alternative"
+
+          part html_part
+          part text_part
+        end
+
+        @message.parts.unshift(content)
+      end
     end
 
     def header_value(name)

spec/components/email/sender_spec.rb

@@ -412,6 +412,16 @@ describe Email::Sender do
       expect(message.attachments.map(&:filename))
         .to contain_exactly(*[small_pdf, csv_file].map(&:original_filename))
     end
+
+    it "structures the email as a multipart/mixed with a multipart/alternative first part" do
+      SiteSetting.email_total_attachment_size_limit_kb = 10_000
+      Email::Sender.new(message, :valid_type).send
+
+      expect(message.content_type).to start_with("multipart/mixed")
+      expect(message.parts.size).to eq(4)
+      expect(message.parts[0].content_type).to start_with("multipart/alternative")
+      expect(message.parts[0].parts.size).to eq(2)
+    end
   end
 
   context 'with a deleted post' do