From https://once.com/writebook

test/controllers/action_text/markdown/uploads_controller_test.rb

@@ -0,0 +1,31 @@
+require "test_helper"
+
+class ActionText::Markdown::UploadsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    sign_in :kevin
+  end
+
+  test "attach a file" do
+    assert_changes -> { ActiveStorage::Attachment.count }, 1 do
+      post action_text_markdown_uploads_url, params: {
+        record_gid: pages(:welcome).to_signed_global_id.to_s,
+        attribute_name: "body",
+        file: fixture_file_upload("reading.webp", "image/webp")
+      }, as: :xhr
+    end
+
+    assert_response :success
+  end
+
+  test "view attached file" do
+    markdown = pages(:welcome).body.tap(&:save!)
+    markdown.uploads.attach fixture_file_upload("reading.webp", "image/webp")
+
+    attachment = pages(:welcome).body.uploads.last
+
+    get action_text_markdown_upload_url(slug: attachment.slug)
+
+    assert_response :redirect
+    assert_match /\/rails\/active_storage\/.*\/reading\.webp/, @response.redirect_url
+  end
+end