Remove hack to allow IO files in Active Storage

We were using this hack in order to allow `File.new` attachments in
tests files. However, we can use the `fixture_file_upload` helper
instead.

Just like it happened with `file_fixture`, this helper method doesn't
work in fixtures, so in this case we're using `Rack::Test::UploadedFile`
instead.

spec/lib/tasks/active_storage_spec.rb

@@ -35,7 +35,7 @@ describe "active storage tasks" do
     end
 
     it "does not modify old ckeditor attachments" do
-      image = Ckeditor::Picture.create!(data: Rack::Test::UploadedFile.new(file_fixture("clippy.png")))
+      image = Ckeditor::Picture.create!(data: fixture_file_upload("clippy.png"))
 
       expect(image.storage_data.attachment.name).to eq "storage_data"