what, is it good for? absolutely nothing

before_filter :csv?, :only => :index

def index
  respond_to do |format|
    format.html { @users = User.all }
    format.csv { @users = User.to_csv }
  end
end

def csv?
  if request.format == :csv && ! logged_in?
    log_in
  end
end

def csv?
  if request.format.csv? && ! logged_in?
    log_in
  end
end

Introducing What

What is a plugin that automatically adds those nice query methods to ActionController::Mime::Type, the type of object returned by Request#format. All the built in mime types are supported. Any custom mime types that you add will also have corresponding query methods.