shrine_61.rb

require 'shrine'
require 'shrine/storage/memory'

Shrine.storages = {
  cache: Shrine::Storage::Memory.new,
  store: Shrine::Storage::Memory.new
}

class DocumentUploader < Shrine
  plugin :determine_mime_type
  plugin :validation_helpers

  Attacher.validate do
    validate_mime_type_inclusion ['image/jpeg']
  end
end

class Document
  include DocumentUploader[:attachment]
  attr_accessor :attachment_data
end

document = Document.new
document.attachment = File.open('-image.jpg')

p document.attachment_attacher.errors