In Powermail 1.x i could set this feature in the Powermail Constants:
# cat=powermail uploads/enable/0530; type=boolean; label= Delete after attach: If checked, all uploaded files will be deleted from the server after the mail was sent
upload.delete = 0
Is it not possible anymore to delete the uploaded files from the server? Is the only way to delete these files with a cronjob?
You do not need a own cronjob.
Go to the scheduler, add a new task, choose "Extbase Command Controller Task".
Open the "CommandController Command" drop down. There you will find the task "powermail: Clean Unused Uploads".