FIX: Inline secure images with duplicated names (#13926)

Inlining secure images with the same name was not possible because they
were indexed by filename. If an email contained two files with the same
name, only the first image was used for both of them. The other file
was still attached to the email.

lib/email/sender.rb

@@ -26,7 +26,8 @@ module Email
   class Sender
 
     def initialize(message, email_type, user = nil)
-      @message =  message
+      @message = message
+      @message_attachments_index = {}
       @email_type = email_type
       @user = user
     end
@@ -240,7 +241,7 @@ module Email
       # Embeds any of the secure images that have been attached inline,
       # removing the redaction notice.
       if SiteSetting.secure_media_allow_embed_images_in_emails
-        style.inline_secure_images(@message.attachments)
+        style.inline_secure_images(@message.attachments, @message_attachments_index)
       end
 
       @message.html_part.body = style.to_s
@@ -328,6 +329,7 @@ module Email
             Discourse.store.download(attached_upload).path
           end
 
+          @message_attachments_index[original_upload.sha1] = @message.attachments.size
           @message.attachments[original_upload.original_filename] = File.read(path)
           email_size += File.size(path)
         rescue => e