code: function uploadSkin ({accessToken}, profileId, file, isSlim = false) {
    const form = new FormData()
    form.append('model', isSlim ? 'slim' : '')
    form.append('file', file)
  
    return fetch(`${CORE_API}/user/profile/${profileId}/skin`, {
      method: 'PUT',
      body: form,
      headers: {
        'user-agent': USER_AGENT,
        'authorization': `Bearer ${accessToken}`
      }
    })
      .then(handleErrors)
      .then(res => null)
  }
docs: Uploads a file as a logged-in user's in-game skin.
  
  @param {Object} session - object from authentication
  @param {string} session.accessToken - valid access token for the user's account
  @param {string} profileId - which profile UUID to update the skin of
  @param {Stream|Buffer} file - new skin's binary image data
  @param {boolean} [isSlim] - if true, the slim model will be used with the skin
  @returns {Promise} resolves if access token, profile UUID, and stream are valid
  @see {@link http://wiki.vg/Mojang_API#Change_Skin}