farm-2.x-dev/modules/core/asset/asset.links.action.yml

entity.asset.add_page:
  route_name: 'entity.asset.add_page'
  title: 'Add asset'
  appears_on:
    - entity.asset.collection

entity.asset_type.add_form:
  route_name: 'entity.asset_type.add_form'
  title: 'Add asset type'
  appears_on:
    - entity.asset_type.collection