example.rb

class ClassificationSet < ActiveRecord::Base
  mount_uploader :reference_file, ReferenceFileUploader

  validates :reference_file, :presence => { :message => "must have a file selected" }
end

Factory.define :classification_set do |c|
  c.images_zip { File.open(Rails.root.join('spec', 'file_fixtures', 'images.zip')) }
  c.reference_file { File.open(Rails.root.join('spec', 'file_fixtures', 'reference_file.txt')) }
end

Factory(:classification_set)

# Failure/Error: classification_set = Factory(:classification_set)
#   ActiveRecord::RecordInvalid:
#   Validation failed: Reference file must have a file selected