Thumbnail Metadata Disappeared When Importing File with Copy to App

This is a repost from StackOverflow.

I have been using the URL Resources to embed the thumbnail metadata into my custom document-based file. When I export my custom document file, the thumbnail shows up nicely when glanced in iOS Files app.

    override func fileAttributesToWrite(to url: URL, for saveOperation: UIDocumentSaveOperation) throws -> [AnyHashable : Any] {
        var attr: [AnyHashable : Any] = [URLResourceKey.hasHiddenExtensionKey: true]
      
        // Ignore the proper resizing for now.
        ...
      
        attr[URLResourceKey.thumbnailDictionaryKey] = [
            URLThumbnailDictionaryItem.NSThumbnail1024x1024SizeKey: #imageLiteral(resourceName: "Star")
        ]
      
        return attr
    }

Image 1

(Icon Credit: Use Your Loaf)

However, when I import the file back into my app with Share action, `Copy to <MyApp>`, all the metadata seems to survive except the thumbnail.

Image 2

    func application(_ app: UIApplication, open url: URL, options: [UIApplicationOpenURLOptionsKey : Any] = [:]) -> Bool {
      
        // Other non-image metadata survives, like creationDate.
        if let creationDateOpt = try? url.resourceValues(forKeys: [.creationDateKey]).creationDate,
            let creationDate = creationDateOpt {
            print("creationDate: \(creationDate)")
        }
      
      
        if let thumbnailDictOpt = try? url.resourceValues(forKeys: [.thumbnailDictionaryKey]).thumbnailDictionary,
            let thumbnailDict = thumbnailDictOpt,
            let thumbnail = thumbnailDict[URLThumbnailDictionaryItem.NSThumbnail1024x1024SizeKey] {
            print ("Thumbnail survived. Size: \(thumbnail.size)")
            return true
        }
        print ("Thumbnail disappeard!")
        return false
    }

Considering that the thumbnail persists when I manual copy the file in Files app, the lost must happen during the system copying my file from Files to my app's container. Why does this happen? I can manually embed thumbnail as part of my file's data too, but I feel there should be a way to solve this, since other text-based metadata persists.

I tried using `promisedResourceValue` method instead, suspecting that the file might not be fully copied when it's opened, but the result is the same.

Image 3

I have my full project available here on GitHub: https://github.com/pt2277/TestUIDocumentSave