Add helper copy to admin attachments interface

Attachments don't work consistently across all formats. While
experienced publishers know this, we sometimes get support requests from
users confused as to why their attachments are not displaying.

This commit aims to improve the situation ever so slightly with some
copy.

Relevant Trello ticket (2ndline):
https://trello.com/c/XeE6aw32/156-improve-whitehall-publisher-attachment-copy

app/views/admin/attachments/index.html.erb

@@ -1,11 +1,20 @@
 <% page_title "Attachments for #{attachment.attachable_model_name}" %>
 <% page_class "attachments index" %>
+<% is_publication = attachment.attachable_model_name == 'publication' %>
 
 <div class="row">
   <section class="col-md-8">
     <h1>Attachments for <%= attachment.attachable_model_name %></h1>
 
     <%= attachable_editing_tabs(attachable) do %>
+      <p class="qa-helper-copy">
+        <strong>Note:</strong>
+        <% if is_publication %>
+          Attachments added to a publication will appear automatically.
+        <% else %>
+          Attachments need to be referenced in the body markdown to appear in your document.
+        <% end %>
+      <p>
       <ul class="actions list-unstyled">
         <li>
           <%= link_to 'Upload new file attachment', new_polymorphic_path([:admin, typecast_for_attachable_routing(attachable), Attachment]) %>

test/integration/attachments_test.rb

@@ -0,0 +1,28 @@
+require 'test_helper'
+require 'capybara/rails'
+
+class AttachmentsTest < ActionDispatch::IntegrationTest
+  include Capybara::DSL
+
+  setup do
+    login_as_admin
+  end
+
+  test 'displays attachment helper copy for non-publications' do
+    edition = create(:edition)
+    visit "/government/admin/editions/#{edition.id}/attachments"
+
+    within ".qa-helper-copy" do
+      assert_text "need to be referenced"
+    end
+  end
+
+  test 'displays different helper copy for publications' do
+    publication = create(:publication)
+    visit "/government/admin/editions/#{publication.id}/attachments"
+
+    within ".qa-helper-copy" do
+      assert_text "will appear automatically"
+    end
+  end
+end