Fix rails#50005 transform_job not accepting previewables

The transform_job crahes if you want to preprocess a previewable files.
This commit fixes that by using the blob's `representation` method to
process variants or previews.

Co-authored-by: Jonathan Hefner <[email protected]>

Author: nicowenterodt

activestorage/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Fix `preprocessed: true` option for named variants of previewable files.
+
+    *Nico Wenterodt*
+
 *   Allow accepting `service` as a proc as well in `has_one_attached` and `has_many_attached`.
 
     *Yogesh Khater*

activestorage/app/jobs/active_storage/transform_job.rb

@@ -7,6 +7,6 @@ class ActiveStorage::TransformJob < ActiveStorage::BaseJob
   retry_on ActiveStorage::IntegrityError, attempts: 10, wait: :polynomially_longer
 
   def perform(blob, transformations)
-    blob.variant(transformations).processed
+    blob.representation(transformations).processed
   end
 end

activestorage/test/jobs/transform_job_test.rb

@@ -16,6 +16,17 @@ class ActiveStorage::TransformJobTest < ActiveJob::TestCase
     end
   end
 
+  test "creates variant for previewable file" do
+    @blob = create_file_blob(filename: "report.pdf", content_type: "application/pdf")
+    transformations = { resize_to_limit: [100, 100] }
+
+    assert_changes -> { @blob.reload.preview(transformations).send(:processed?) }, from: false, to: true do
+      perform_enqueued_jobs do
+        ActiveStorage::TransformJob.perform_later @blob, transformations
+      end
+    end
+  end
+
   test "creates variant when untracked" do
     @was_tracking, ActiveStorage.track_variants = ActiveStorage.track_variants, false
     transformations = { resize_to_limit: [100, 100] }