file_browser-8.x-1.3/file_browser.routing.yml

file_browser.preview:
  path: '/admin/file-browser-preview/{file}/{image_style}'
  defaults:
    _title: 'Preview'
    _controller: '\Drupal\file_browser\Controller\FileBrowserController::preview'
    image_style: ''
  requirements:
    _entity_access: 'file.view'
    file: \d+
  # Who knows why this isn't converted normally...
  options:
    parameters:
      file:
        type: entity:file