How to properly add data files into a SwiftData ModelDocument file package

Hi all, I am working on a macOS/iOS sandboxed app with SwiftUI/SwiftData. The app saves its' data into a ModelDocument file. But I want to also save large binary data files into this file.

I create the DocumentGroup scene with:

    DocumentGroup(editing: Entry.self, contentType: .myDocument) {
        MainWindowView()
    }

In my MainWindowView, I use

@Environment(\.documentConfiguration) private var documentConfiguration

to get the URL to the user created ModelDocument file URL.

When I need to add large binary file into the ModelDocument file, I call a method in my model:

func saveReferencedData(_ data: Data, documentURL: URL?) throws {
    let logger = Logger(subsystem: "saveReferencedData", category: "Asset")
    
    if let documentURL {
        let referencedFileName = "\(entryIdentifier)_\(assetIdentifier).\(assetType)"
        let tempFileURL = documentURL.appending(components: referencedFileName)

        if documentURL.startAccessingSecurityScopedResource() {
            do {
                try data.write(to: tempFileURL, options: [])
            } catch {
                documentURL.stopAccessingSecurityScopedResource()
                throw AssetFileOperationError.unableToSaveReferenceFile
            }
            self.referencedFileLocation = referencedFileName
            
            logger.debug("Successfully saved image data to: \(referenceFileURL)")
        }
        documentURL.stopAccessingSecurityScopedResource()
    } else {
        logger.debug("ERROR! Unable to save referenced image file because document URL is nil.")
    }
}

When this method is called, the data file is saved, but immediately I gets this diablog box:

If I click on any of the options in the dialog box, the binary data file is removed.

I think this is a permission problem and I am not writing to the ModelDocument file correctly. But I can not find any information online to experiment more.

Does anyone have experience with a customized ModelDocument file with more data files written into it? Or if you have any insight or advice, it would be greatly appreciated.

Thank you so much for reading.