Full validation support for Active Storage attachments won’t be arriving until Rails 6. Until then you’ll need to write your own custom validators to add this missing functionality. Last time I showed you how to write an attachment presence validator. Next I’ll share a validator which can restrict the content-type of your attachments. Drop this bit of code and its accompanying RSpec matcher into your project to tide you over until the next Rails release.
http://www.carlosramireziii.com/validating-the-content-type-of-active-storage-attachments.html