private val boundary: ByteString = boundary.encodeUtf8()
      private var type = MIXED
      private val parts = mutableListOf<Part>()

      /**
       * Set the MIME type. Expected values for `type` are [MIXED] (the default), [ALTERNATIVE],
       * [DIGEST], [PARALLEL] and [FORM].
       */
      fun setType(type: MediaType) =
        apply {