How to prevent ios from deleting application generated data file on ipa update.

Hi,
I have this problem: I have an app that when start download from internet a zip file and extract it in a folder like that:

System.IO.Path.Combine(System.Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), ".data");

But if I make an update to the app and reinstall it, I lost all this file and i get Null Exception. In iOS7 i never had this problem. There is some flag to add to this folder to prevent the ios delete the data?

Thanks a lot.
Marco