module Sprockets::Mime

def compute_extname_map
  graph = {}
  ([nil] + pipelines.keys.map(&:to_s)).each do |pipeline|
    pipeline_extname = ".#{pipeline}" if pipeline
    ([[nil, nil]] + config[:mime_exts].to_a).each do |format_extname, format_type|
      4.times do |n|
        config[:engines].keys.permutation(n).each do |engine_extnames|
          key = "#{pipeline_extname}#{format_extname}#{engine_extnames.join}"
          type = format_type || config[:engine_mime_types][engine_extnames.first]
          graph[key] = {type: type, engines: engine_extnames, pipeline: pipeline}
        end
      end
    end
  end
  graph
end