I don’t think that would work to be honest, as there isn’t any way to close the mobile app other than to kill it, which will prevent any task from running. Not to mention that there are already limitations with the mobile app that it cannot do anything in the background, most notably to sync in the background (which is a long time requested feature, but also with no completely reliable solution).
Cleaning on opening the app would probably not be good either as the are already a number of maintanence tasks which run on startup and it’s best not to add another unnecessarily.
A manual delete local resources option should be supposedly more straightforward though
EDIT: Also in your other thread there was mention of concern about deleting .cryted files while the encryption service is running: Purge of .crypted files ? or orphan resources? - #2 by laurent
So that adds complications to having a scheduled task to clear down the resources folder. However if adding a manual action on mobile, in addition to validating attachment behaviour is set to manual or auto to allow you to do the deletion, it could validate that there are no outgoing changes pending to be synced (as the crypted files are only created before uploading an outgoing resource change to be synced, or when downloading a resource, which only happens on demand if attachment behaviour is set to manual or auto). Under those circumstances, it’s only possible that any new changes to files in the resource folder since starting the deletion task could be in use by the encryption service, but a timestamp could be recorded in memory when the task was started, and for each file, check the timestamp is before the start time, and only delete the file if it is