old_contents: import os
  from seahub.utils import get_service_url
  def get_thumbnail_src(repo_id, obj_id, size):
      return os.path.join(get_service_url(), "thumbnail", repo_id,
                          obj_id, size, '')
  
new_contents: import posixpath
  from seahub.utils import get_service_url
  def get_thumbnail_src(repo_id, obj_id, size):
      return posixpath.join(get_service_url(), "thumbnail", repo_id,
                            obj_id, size)
  
subject: use posixpath to generate thumbnail-src