FEATURE: email attachments in a details (#27804)

This change how we present attachments from incoming emails to now be "hidden" in a "[details]" so they don't "hang" at the end of the post.

This is especially useful when using Discourse as a support tool where email is the main communication channel. For various reasons, images are often duplicated by email user agents, and hiding them behind the details block help keep the conversation focused on the isssue at hand.

Internal ref t/122333

lib/email/receiver.rb

@@ -462,6 +462,10 @@ module Email
         end
       end
 
+      # keep track of inlined images in html version
+      # so we can later check if they were elided
+      @cids = (html.presence || "").scan(/src\s*=\s*['"](cid:.+?)["']/).flatten
+
       markdown, elided_markdown =
         if html.present?
           # use the first html extracter that matches
@@ -772,7 +776,7 @@ module Email
           # return the email address and name
           [mail, name]
         end
-      rescue Mail::Field::ParseError, Mail::Field::IncompleteParseError => e
+      rescue Mail::Field::ParseError, Mail::Field::IncompleteParseError
         # something went wrong parsing the email header value, return nil
       end
     end
@@ -1366,7 +1370,21 @@ module Email
 
       upload_shas = Upload.where(id: upload_ids).pluck("DISTINCT COALESCE(original_sha1, sha1)")
 
+      is_duplicate = ->(upload_id, upload_sha, attachment) do
+        return true if upload_id && upload_ids.include?(upload_id)
+        return true if upload_sha && upload_shas.include?(upload_sha)
+
+        if attachment.respond_to?(:url) && attachment.url&.start_with?("cid:") &&
+             attachment.content_type&.start_with?("image/")
+          return true if @cids&.include?(attachment.url)
+        end
+
+        false
+      end
+
+      added_attachments = []
       rejected_attachments = []
+
       attachments.each do |attachment|
         tmp = Tempfile.new(["discourse-email-attachment", File.extname(attachment.filename)])
         begin
@@ -1392,11 +1410,11 @@ module Email
                 end
               elsif raw[/\[image:[^\]]*\]/i]
                 raw.sub!(/\[image:[^\]]*\]/i, UploadMarkdown.new(upload).to_markdown)
-              elsif !upload_ids.include?(upload.id) && !upload_shas.include?(upload_sha)
-                raw << "\n\n#{UploadMarkdown.new(upload).to_markdown}\n\n"
+              elsif !is_duplicate[upload.id, upload_sha, attachment]
+                added_attachments << upload
               end
-            elsif !upload_ids.include?(upload.id) && !upload_shas.include?(upload_sha)
-              raw << "\n\n#{UploadMarkdown.new(upload).to_markdown}\n\n"
+            elsif !is_duplicate[upload.id, upload_sha, attachment]
+              added_attachments << upload
             end
           else
             rejected_attachments << upload
@@ -1406,10 +1424,24 @@ module Email
           tmp&.close!
         end
       end
+
       if rejected_attachments.present? && !user.staged?
         notify_about_rejected_attachment(rejected_attachments)
       end
 
+      if added_attachments.present?
+        markdown =
+          added_attachments.map { |upload| UploadMarkdown.new(upload).to_markdown }.join("\n")
+        if markdown.present?
+          raw << "\n\n"
+          raw << "[details=\"#{I18n.t("emails.incoming.attachments")}\"]"
+          raw << "\n\n"
+          raw << markdown
+          raw << "\n\n"
+          raw << "[/details]"
+        end
+      end
+
       raw
     end