As I understand Herokus Ephemeral filesystem will only allow you write access to a temporary folder. Created files in that folder will be discarded when the dyno the files w
Dynos are cycled every 24 hours so your files will never last longer than that.
Therefore, it's safe to say that you don't need to worry about archiving these files off.
Note; just a small thing. Cedar lets you write to anywhere on the dyno. Bamboo only lets you write to /tmp.