The user would then use a terminal emulator to connect to something like a BBS[1] where they could browse files, and download or upload files to the connected USB storage device using XMODEM[2] like in the good old days.
edit: It could of course also filter the files, for example not list any executable files, and prevent transfer of executable files based on scanning their contents.
The MITM device would be implemented using a microcontroller with signed firmware, and careful design to prevent a connected USB device to do shenanigans like voltage glitching. This would include using isolated DC-DC and isolated data lines ala like this[3].
The MITM would only interact with the storage device class. If the connected device presents itself as more, say a keyboard, it would just ignore those.
The user must be prevented from bypassing the MITM device, though this could be done through physical means.
[1]: https://en.wikipedia.org/wiki/Bulletin_board_system
[2]: https://en.wikipedia.org/wiki/XMODEM
[3]: https://ez.analog.com/ez-blogs/b/engineerzone-spotlight/post...