Apple的ShapeEdit示例有一个缩略图缓存，使用NSURLDocumentIdentifierKey键入。参见
https://developer.apple.com/library/content/samplecode/ShapeEdit/Introduction/Intro.html https://developer.apple.com/library/content/samplecode/ShapeEdit/Listings/ShapeEdit_DocumentBrowser_ThumbnailCache_swift.html
只有文档在iCloud中时才会出现此密钥。应该使用哪种密钥来缓存本地文档上的缩略图？