In order to view midi files while browsing, you have to start with an option on the keyboard that is related to midi files.
example : on the main menu (on the touch screen) above the style and multipad, you have 'songA' and 'song B'. Press one of them, then check in the top left corner of the screen if MIDI is chosen (not song - that would be for mp3 's or so) and then - with MIDI chosen - , access your usb drive and navigate to the folder where your midi files are. You should now see them.
Best regards
Rudy