Fix attachment not being re-downloaded even if file is not stored (#12125)

Change the behaviour of remotable concern. Previously, it would skip
downloading an attachment if the stored remote URL is identical to
the new one. Now it would not be skipped if the attachment is not
actually currently stored by Paperclip.

app/models/concerns/remotable.rb

@@ -18,7 +18,7 @@ module Remotable
           return
         end
 
-        return if !%w(http https).include?(parsed_url.scheme) || parsed_url.host.blank? || self[attribute_name] == url
+        return if !%w(http https).include?(parsed_url.scheme) || parsed_url.host.blank? || (self[attribute_name] == url && send("#{attachment_name}_file_name").present?)
 
         begin
           Request.new(:get, url).perform do |response|