Apple does not allow the use of the iPhone file system stock. You'd need access to that do what you want.
If you jailbreak, there's a way to do it, but most of the tweaks for that broke with the iOS 9 update and they haven't been updated. There may be some out there that I am unaware of but with the semi-untethered nature of the latest jailbreaks I don't have much hope that they would work.
Oh, and you'd need to be on a jailbreakable firmware anyway.
There are PC/Mac apps out there (such as Phoneview for the Mac) that give limited access but any transferring would have to be done through that app.
None of those apps are free.