add a new es_systems.cfg for say “media files” point it to a location with some files (maybe a remote mount or something), and use something like omxplayer ? it could then just use the emulation station browser to show a list of media to play.

alternatively you could make a little script you launch that uses something like

FILE=$(dialog --stdout --title "Please choose a file" --fselect $HOME/ 20 60)

to get a file path via a little file browser

http://askubuntu.com/questions/488350/how-to-prompt-users-with-a-gui-dialog-box-to-choose-file-directory-path-via-the