Temporary files are not deleted

Problem/Motivation

Hi !
If someone starts to upload encrypted files, a directory "_sid_" is created and files are moved to the real directory after saving form.
But, If the form is not submitted or the user cancel before saving, files in "_sid_" are still there.

Steps to reproduce

Just start uploading files into encrypted file field and refresh form's page. Temporary files are still there.

Proposed resolution

These files should be deleted on cron job or when deleting caches.
Maybe a lock system should be implemented, associated with the form being filled, to avoid deleting files currently in use.