I still say I don't think this is possible. As I know it, the whole point of Spotify is the ability to access files from your computer remotely, most notably, via mobile. The setting your talking about in the phone settings is to change how a PC sees the device when you connect it to USB. So if you connected the phone to a laptop, have the laptop reconise the phone as a USB drive, sync the drive with the laptop's spotify account, and then access that account on the phone via the spotify app. The reason you can see a drive on one computer from another computer is because you have File and network sharing enabled on the two computers in the LAN. File and network sharing works through a protocal by the same name installed on your NIC driver, just like TCP or a AV firewall. The phone has not such protocal, unless through the spotify app. And if that is a case, your going to want to look through the App for the setting, not the phone. Make sense?
What you probably want to do is use the USB cord to transfer the music from the phone to the desktop, sync that with your Spotify account, and then access that account from your phone.