code: def _rejoin_mimebundle(data):
      """Rejoin the multi-line string fields in a mimebundle (in-place)"""
      for key, value in list(data.items()):
          if (
              not _is_json_mime(key)
              and isinstance(value, list)
              and all(isinstance(line, str) for line in value)
          ):
              data[key] = "".join(value)
      return data
  
file_path: nbformat/nbformat/v4/rwbase.py
imports: from __future__ import annotations
label_desc: Use image node_id 2 for calling a global function with example usage: _rejoin_mimebundle(data) and returns: data