docs: def parse_header_tags():
      """Parse tags specified in the HTTP request header."""
      # Get the value of the custom HTTP header and interpret it as an query
      # string
      qs = request.headers.get(
          current_app.config['FILES_REST_FILE_TAGS_HEADER'], '')
  
      tags = {}
      for key, value in parse_qsl(qs):
          # Check for duplicate keys
          if key in tags:
              raise DuplicateTagError()
          # Check for too short/long keys and values.
          if not validate_tag(key, value):
              raise InvalidTagError()
          tags[key] = value
      return tags or None