Error in generating extension for `video/mp4` content type for ActiveStorage image delivery url

**Problem**

When using with Rails ActiveStorage, the [extension generated](https://github.com/cloudinary/cloudinary_gem/blob/master/lib/active_storage/service/cloudinary_service.rb#L157) for a content_type of `video/mp4` is `mp4v`, while it is saved in cloudinary with the extension of `mp4` in the url. As a result, [image delivery](https://cloudinary.com/documentation/rails_activestorage#image_delivery) with Active Storage is broken.

Extension generated should be `mp4`.