You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: books_helper.rb
    path of file: ./repos/website/app/helpers
    
    the code of the file until where you have to start completion: module BooksHelper
  def thumbnail_link_to_large_image small
    
  def extension_for_ebook type
    case type
    when :epub
      'epub'
    when :mobi
      'mobi'
    else
      'pdf'
    end