module Sprockets::Mime

def compute_extname_map
  graph = {}
  ([[nil, nil]] + mime_exts.to_a).each do |format_extname, format_type|
    3.times do |n|
      engines.keys.permutation(n).each do |engine_extnames|
        key = "#{format_extname}#{engine_extnames.join}"
        type = format_type || engine_mime_types[engine_extnames.first]
        graph[key] = {type: type, engines: engine_extnames}
      end
    end
  end
  graph
end