This is a security nightmare. It looks like major browser vendors have pushed back against it and for good reason. Even a website I absolutely trust should not be able to access my bare filesystem because I don't know if that website will get compromised, XSS'd, DNS poisoned, etc.

I understand wanting to save files with a webapp, but why not just use LocalStorage and deserialize/serialize from that? If you need persistence plug in a cloud solution like Amazon S3. That's what the web is for.