addPart(Part.createFormData(name, filename, body))
      }

      /** Add a part to the body. */
      fun addPart(part: Part) =
        apply {
          parts += part
        }

      /** Assemble the specified parts into a request body. */
      fun build(): MultipartBody {
        check(parts.isNotEmpty()) { "Multipart body must have at least one part." }