in _plugins/read.rb [40:50]
def load_file(context, filename, context_page)
file = Pathname.new(File.dirname(context_page['path'])) + filename
file = file.cleanpath
file = file.relative_path_from(Pathname.new("/")) unless file.relative?
raise "No such file #{file} in read call (from #{context_page['path']})" unless file.exist?
file
end