gistfile1.rb

class Song < ActiveRecord::Base
  belongs_to :session
  
  validates_presence_of :file_name
  validate :file_name_has_to_be_audio

  private

    def file_name_has_to_be_audio
      unless ['mp3', 'flac', 'wav'].include? file_name.split(".").last.downcase
        errors.add(:file_name, "has to be mp3/flac/wav file")
      end
    end
end