# File lib/aws/s3/presigned_post.rb, line 342
      def fields

        signature = config.signer.sign(policy, "sha1")

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

        fields["x-amz-security-token"] = config.signer.session_token if
          config.signer.session_token

        fields.merge(optional_fields)

      end