Impossible to manually add multi-file games
Some examples of such games include bin+cue images, games that have multiple disks, and DOS games.
File chooser portal doesn't return real file path, but something like file:///run/user/1000/doc/f3e15ed/Pac-Man%20(USA).a26
, where f3e15ed
part is different from each file, and mounts the selected file there without any other files in the same directory. This leads to a situation when a user selects one file of a multi-file game, and Games only sees that one file, so the game either doesn't show up or doesn't run, even if the whole directory is visible from sandbox.