Keep VariantWithRecord API consistent with Variant

Similar to c18bcd5 adds the
content_type and filename methods to VariantWithRecord.

activestorage/app/models/active_storage/variant_with_record.rb

@@ -7,6 +7,7 @@
 class ActiveStorage::VariantWithRecord
   attr_reader :blob, :variation
   delegate :service, to: :blob
+  delegate :content_type, to: :variation
 
   def initialize(blob, variation)
     @blob, @variation = blob, ActiveStorage::Variation.wrap(variation)
@@ -21,6 +22,10 @@ def image
     record&.image
   end
 
+  def filename
+    ActiveStorage::Filename.new "#{blob.filename.base}.#{variation.format.downcase}"
+  end
+
   # Destroys record and deletes file from service.
   def destroy
     record&.destroy

activestorage/test/models/variant_with_record_test.rb

@@ -23,6 +23,8 @@ class ActiveStorage::VariantWithRecordTest < ActiveSupport::TestCase
     end
 
     assert_match(/racecar\.jpg/, variant.url)
+    assert_equal "racecar.jpg", variant.filename.to_s
+    assert_equal "image/jpeg", variant.content_type
 
     image = read_image(variant.image)
     assert_equal 100, image.width