Upgrade Paperclip 3x to 4x

What should be considered when upgrading to Paperclip 4.x

Starting with Paperclip version 4.0, all attachments are required to include a content_type validation, a file_name validation, or to explicitly state that they're not going to have either.

Paperclip raises Paperclip::Errors::MissingRequiredValidatorError error if you do not do any of this.

http://stackoverflow.com/questions/21897725