module ActionView::Helpers::AssetUrlHelper

def compute_asset_extname(source, options = {})

nothing should be added.
Compute extname to append to asset path. Returns nil if 
def compute_asset_extname(source, options = {})
  return if options[:extname] == false
  extname = options[:extname] || ASSET_EXTENSIONS[options[:type]]
  extname if extname && File.extname(source) != extname
end