Implement ActiveStorage::Blob#attachable_plain_text_representation

Fixes that file attachments without captions would not be represented in plain text generated from rich-text content, causing ActionText::RichText#present? to return false.

Closes #36607.

actiontext/lib/action_text/engine.rb

@@ -25,6 +25,10 @@ module ActionText
         def previewable_attachable?
           representable?
         end
+
+        def attachable_plain_text_representation(caption = nil)
+          "[#{caption || filename}]"
+        end
       end
     end
 

actiontext/test/unit/attachment_test.rb

@@ -50,6 +50,11 @@ class ActionText::AttachmentTest < ActiveSupport::TestCase
     assert_not_nil trix_attachment.attributes["content"]
   end
 
+  test "converts to plain text" do
+    assert_equal "[Vroom vroom]", ActionText::Attachment.from_attachable(attachable, caption: "Vroom vroom").to_plain_text
+    assert_equal "[racecar.jpg]", ActionText::Attachment.from_attachable(attachable).to_plain_text
+  end
+
   test "defaults trix partial to model partial" do
     attachable = Page.create! title: "Homepage"
     assert_equal "pages/page", attachable.to_trix_content_attachment_partial_path