Re Umbraco forms and the temp folder storage..
I believe it’s part of the serverside validation capability of UCMS and UForms to allow you to virus check/other process the file in a safe temp folder before copying over into less secure storage. (so any backoffice fileupload and also frontend fileuploads in UForms)
Some discussion here.. Umbraco Forms, Upload File.. any core protections against common exploits? - Umbraco community forum