service.rb

# 'html' is the default when there is no extension or Accept header
# we will actually default to json, though ...
Formats = [ :json, :html ]

on( :get, [ :resource ] ) do 
  # ... do something to get results
  format = File.extname( path ); format = 'json' if format.empty?
  results.send( "to_#{ format }" )
end