Fix #4987: Can't populate tag string from upload url anymore.

Usage: https://danbooru.donmai.us/uploads/new?url=...&post[tag_string]=...&post[rating]=...

* Pass the URL parameters from the /uploads/new page to the /uploads/:id page.
* Fix the /uploads/:id page throwing an "unpermitted parameters" error
  when given URL params for the post edit form.

app/policies/post_policy.rb

@@ -76,6 +76,12 @@ class PostPolicy < ApplicationPolicy
     translated_commentary_desc translated_commentary_title]
   end
 
+  # XXX For UploadsController#show action
+  def permitted_attributes_for_show
+    %i[tag_string rating parent_id source is_pending artist_commentary_desc
+    artist_commentary_title translated_commentary_desc translated_commentary_title]
+  end
+
   def permitted_attributes_for_update
     %i[tag_string old_tag_string parent_id old_parent_id source old_source rating old_rating has_embedded_notes]
   end