# File lib/aws/s3/presigned_post.rb, line 328
      def fields
        signature =
          config.signer.sign(policy, "sha1")

        {
          "AWSAccessKeyId" => config.signer.access_key_id,
          "key" => key,
          "policy" => policy,
          "signature" => signature
        }.merge(optional_fields)
      end