Override `file_storage` on the `ObjectStoreUploader`

Normally the storage-stype of an uploader is set on the class level of
an uploader [0] and the `storage` property is initialized from there [1].

In a security release, we were using that at the class level to
determine the path of an upload and whether or not to move it.

In the `ObjectStoreUploader` the storage method is overridden, meaning
that the storage-type set on the class could be different from the
actual storage used.

[0] https://github.com/carrierwaveuploader/carrierwave/blob/f2f8569b1013f601fe7be4b3a616359acd27240d/lib/carrierwave/uploader/configuration.rb#L8
[1] https://github.com/carrierwaveuploader/carrierwave/blob/f2f8569b1013f601fe7be4b3a616359acd27240d/lib/carrierwave/uploader/store.rb#L88

app/uploaders/object_store_uploader.rb

@@ -112,6 +112,10 @@ class ObjectStoreUploader < GitlabUploader
     raise 'Object Storage feature is missing' unless subject.project.feature_available?(:object_storage)
   end
 
+  def file_storage?
+    storage.is_a?(CarrierWave::Storage::File)
+  end
+
   private
 
   def set_default_local_store(new_file)