NSFilePromiseProvider that supports additional types

ExtendableFilePromiseProvider.swift

class ExtendableFilePromiseProvider: NSFilePromiseProvider {
    var additionalItems: [NSPasteboard.PasteboardType: Any] = [:]

    override func writableTypes(for pasteboard: NSPasteboard) -> [NSPasteboard.PasteboardType] {
        var types = super.writableTypes(for: pasteboard)
        if self.additionalItems.count > 0 {
            types.append(contentsOf: self.additionalItems.keys)
        }
        return types
    }

    override func pasteboardPropertyList(forType type: NSPasteboard.PasteboardType) -> Any? {
        if let item = self.additionalItems[type] {
            return item
        }
        return super.pasteboardPropertyList(forType: type)
    }
}