Re: USB charging mode vs USB file transfer mode -- you should be able to change this by enabling the "developer options" menu. This menu is hidden by default, but you can enable by going to Settings/About Phone and then tapping on "Build Number" about 10 times or so. Then click the back arrow and you will see a new purple "Developer Options" menu mixed in with all the green menus under Settings - System. Click to open the Developer Options menu.
Once in Developer Options, go to Networking -- USB Configuration and you should be able to select from charging, MTP, PTP, RNDIS, Audio Source, and MIDI. As you indicated, the default mode is charging on my SM-910V.