A file explorer attached to the side would be a significant effort. The current file browser was based off an open source library that has been continuously modified for MobileSheetsPro. It's based around having a completely separate activity that starts a foreground service to asynchronously handle displaying the list of files, whether it be from a local storage location or the cloud. In order to attach a file explorer like what you are describing, I would either have to copy the existing file browser and modify it to work as a fragment that can support drag & drop to other parts of the screen (no idea how difficult that is or if it's possible with the Android framework), or implement something completely new which would take a long time. I'm not saying it can't be done, I just think there are other things I should prioritize right now as there are many other ways to import files into MobileSheetsPro right now. I'm also not sure how feasible it is to even display an attached file browser as most tablets are around 10 inches, and that doesn't leave much room for a functional browser.
Mike
Mike