Strip Attachment elements from sidebar

The count of elements is (or at least can be) different depending on the
email headers - like reply-to, etc… So rather than relying on the count
and stripping the last two, find the `<dt>` containing the
"Attachments:" label text, and then remove it and the element following
it.

.rubocop_todo.yml

@@ -18,3 +18,7 @@ Style/Documentation:
     - 'lib/letter_opener_web.rb'
     - 'lib/letter_opener_web/delivery_method.rb'
     - 'lib/letter_opener_web/engine.rb'
+
+Metrics/ClassLength:
+  Exclude:
+    - 'app/models/letter_opener_web/letter.rb'

CHANGELOG.md

@@ -1,3 +1,7 @@
+## [Unreleased](https://github.com/fgrehm/letter_opener_web/compare/v2.0.0...master)
+
+  - Reliably strip Attachment links from the sidebar. [#132](https://github.com/fgrehm/letter_opener_web/pull/132)
+
 ## [v2.0.0](https://github.com/fgrehm/letter_opener_web/compare/v1.4.1...v2.0.0)
 
   - Require Rails >= 5.2, run tests against Rails 6.1 [#113](https://github.com/fgrehm/letter_opener_web/pull/113)

app/models/letter_opener_web/letter.rb

@@ -81,10 +81,13 @@ def valid?
 
     def remove_attachments_link(headers)
       xml = REXML::Document.new(headers)
-      if xml.root.elements.size == 10
-        xml.delete_element('//dd[last()]')
-        xml.delete_element('//dt[last()]')
+      label_element = xml.root.elements.find { |e| e.get_text&.value&.match?(/attachments:/i) }
+
+      if label_element
+        xml.root.delete_element(label_element.next_element) # the list of attachments
+        xml.root.delete_element(label_element)
       end
+
       xml.to_s
     end