Doing this seems a bit counterproductive, Version 7.0.0+ has automatic preload , it's completely plug and play, there's no need to specify files anymore. This means, that as you playtest your game, any audio files, or image files, that are loaded mid scene/map, are automatically flagged for preload without you having to specify them. The next time you enter that scene/map those files will be loaded into memory before the scene ever starts.
This section that you're in is only to load files that should stay in memory at all times, I.E. a file that's used in almost every scene, and or is called frequently, that way time isn't wasted unloading, and reloading the same file between scene transfers.
Imo though, this feature shouldn't be used for entire folders, as that would mean that the entire contents of the folder will always be retained in memory and will never clear itself.
Also folders can't be specified here, only singular files, as it's not intended to be used for large amounts of content for the above mentioned reasons.