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

      /** Add a part to the body. */