function bodyMimeType (object) {
  const { headersList } = object[kState]
  const contentType = headersList.get('content-type')

  if (contentType === null) {
    return 'failure'
  }

  return parseMIMEType(contentType)
}
@see https://fetch.spec.whatwg.org/#concept-body-mime-type
@param {import('./response').Response|import('./request').Request} object