code: function identifyBoundary(header) {
    const parts = header.split('\r\n');
  
    for (let i = 0; i < parts.length; i++) {
      if (parts[i].substr(0, 2) === '--') {
        return parts[i];
      }
    }
  }
docs: Identifies the boundary in a multipart/related message header.
  @param {String} header message header
  @returns {String} boundary