So you can't use other file providers? That tech is really stable now, I've been using it for years to edit a git repo inside Working Copy or files inside other apps using yet another set of apps (Textastic, iA Writer, even vim inside a-Shell). You should really consider reviewing that viewpoint.
As someone who left iOS for Android because of that, I was pleasantly surprised - when I went back - to find that iOS now has a Files app for managing files on the device and also network shares.
iOS has definitely improved in that area and definitely now has a filesystem.