Well that's unfortunate. Currently off the top of my head, I can only think of two ways to access your files on the phone at this point.
One would be to replace the screen. It is not free, takes time and effort, but if you plan to use the 640 anyway, that would be the best option.
Two would be to use Windows Phone Internals 2.4 to enable mass storage mode on the 640. However this may or may not work depending on what Windows 10 Mobile OS version is installed. This method is risky of course as it requires boot loader unlocking the phone, but if it works and you get your files off the phone you can reverse it and lock the boot loader back.
Maybe wait to see if someone else shares a better approach. I am not certain of another way myself though.