Fix embedded attachment processing

For attachments which are embedded within a RFC822 attachment, were
unable process some attachment if they were received a long time ago.

This is down to changes in the underlying mail gem which results in
slight difference in the headers which get appended to the attachment
body.

This change fixes this by only matching against the body of embedded
attachments, ignoring headers and any slight changes.

Fixes #7876

lib/mail_handler/backends/mail_backend.rb

@@ -363,13 +363,13 @@ def extract_attached_message_headers(leaf)
 
       # Generate a hash of the attributes associated with each significant part
       # of a Mail object
-      def get_attachment_attributes(mail)
+      def get_attachment_attributes(mail, extract_headers: true)
         get_attachment_leaves(mail).map do |leaf|
           body = get_part_body(leaf)
 
           if leaf.within_rfc822_attachment
             within_rfc822_subject = get_within_rfc822_subject(leaf)
-            body = extract_attached_message_headers(leaf)
+            body = extract_attached_message_headers(leaf) if extract_headers
           end
 
           leaf_attributes = { url_part_number: leaf.url_part_number,
@@ -394,7 +394,7 @@ def attachment_body_for_hexdigest(mail, hexdigest:)
       end
 
       def attempt_to_find_original_attachment_attributes(mail, body:, nested: false)
-        all_attributes = get_attachment_attributes(mail)
+        all_attributes = get_attachment_attributes(mail, extract_headers: false)
 
         attributes = all_attributes.find do |attrs|
           # ensure both bodies have the same line endings
@@ -409,9 +409,10 @@ def attempt_to_find_original_attachment_attributes(mail, body:, nested: false)
 
         return attributes if nested
 
+        mail_body = Mail.new(body).body.to_s
         attributes ||= attempt_to_find_original_attachment_attributes(
-          mail, body: Mail.new(body).to_s, nested: true
-        )
+          mail, body: mail_body, nested: true
+        ) unless mail_body.empty?
 
         attributes
       end

spec/lib/mail_handler/backends/mail_backend_spec.rb

@@ -437,15 +437,16 @@
     let(:mail) do
       mail_attachment = Mail.new(
         <<~EML
-          Date: Tue, 08 Aug 2023 10:00:00 +0000
-          Message-ID: <64d611ca31906_ccf71e5039542@localhost>
+          Subject: Attached email
+
+          Hello world
         EML
       ).to_s
 
       Mail.new do
         add_file filename: 'crlf.txt', content: "foo\r\nfoo"
         add_file filename: 'lf.txt', content: "bar\nbar"
-        add_file filename: 'mail.txt', content: mail_attachment
+        add_file filename: 'mail.eml', content: mail_attachment
       end
     end
 
@@ -465,14 +466,15 @@
       it { is_expected.to include(body: "bar\nbar") }
     end
 
-    context 'when body missing leading zero on dates' do
+    context 'when attached email headers are different' do
       let(:body) do
         <<~EML
-          Date: Tue, 8 Aug 2023 10:00:00 +0000
-          Message-ID: <64d611ca31906_ccf71e5039542@localhost>
+          Subject: A different subject
+
+          Hello world
         EML
       end
-      it { expect(attributes[:body]).to include('08 Aug 2023') }
+      it { is_expected.to include(body: "Hello world\n") }
     end
 
     context 'when body does not match' do