How to list the uploaded files (image file) of unsaved nodes?
The short answer is that there isn't really a way to do this.

The longer version is that you could make some SQL to find all candidates. The issue is that you can see if a file is used somewhere (on entites) on the file_usage table, but some files wont be "used in that manor. This means that finding a file "not being used" is not the same as it's safe to delete it.

A solution could be to make a cron job, that only checks images that has just been uploaded, but you still have the issue that you might end up deleting files that shouldn't have been deleted.