see SolidOS/folder-pane#5 from @timbl

resource-mapper is creating the file foo.mp3 as foo.mp3$.mp3
contentType of .mp3 is audio/mpeg but iana extension for audio/mpeg is mpga.

So the check is a false wrong match and resource-mapper is creating foo.mp3$.mp3
I think it should be foo.mp3

It can be generalized so that resource-mapper.mapUrlToFile returns foo.ext and not foo.ext$.ext