validators.rb

# Helpful stuff from: http://chrisblunt.com/blog/2009/04/18/rails-writing-dry-custom-validators/

ActiveRecord::Base.class_eval do
  def self.validates_as_file(*attr_names)
    options = {:max_size => 1, :extensions => '.txt'}.merge(attr_names.extract_options!)    
    options[:extensions] = [options[:extensions]] unless options[:extensions].is_a?(Array)
    
    validates_each(attr_names, options) do |record, attr_name, value|     
      unless value.is_a?(Tempfile)
        record.errors.add(attr_name, 'must be a file')
        next
      end
      
      unless value.original_filename =~ Regexp.new('\.(' + options[:extensions].join('|') + ')$')
        record.errors.add(attr_name, 'must have one of the following extensions: ' + options[:extensions].join(', '))
      end
      
      if value.size > options[:max_size].megabytes
        readable_size = options[:max_size].to_s + ' Mb'
        record.errors.add(attr_name, 'must not be greater than ' + readable_size)
      end
    end
  end
end