code: def _normalize_file_extensions(exts):
      """Normalizes a list of extensions for use in filtering.
  
      Each extension is returned from `exts` as lower-case and with a
      leading dot. This format can be used to test an lower-case suffix.
      """
      return ["." + ext.lower() for ext in (exts or [])]
  
imports: import os
label_desc: Use image node_id 5 for calling a global function with example usage: _normalize_file_extensions(exts) and returns: unknown