try_precompressed_files

# File actionpack/lib/action_dispatch/middleware/static.rb, line 122
      def try_precompressed_files(filepath, headers, accept_encoding))
        each_precompressed_filepath(filepath) do |content_encoding, precompressed_filepath|
          if file_readable? precompressed_filepath
            # Identity encoding is default, so we skip Accept-Encoding negotiation and
            # needn't set Content-Encoding.
            #
            # Vary header is expected when we've found other available encodings that
            # Accept-Encoding ruled out.
            if content_encoding == "identity"
              return precompressed_filepath, headers
            else
              headers[ActionDispatch::Constants::VARY] = "accept-encoding"

              if accept_encoding.any? { |enc, _| /\b#{content_encoding}\b/.match?(enc) }
                headers[ActionDispatch::Constants::CONTENT_ENCODING] = content_encoding
                return precompressed_filepath, headers
              end
            end
          end
        end
      end