Images of previewed (not saved) nodes are saved too

In addition to my other issue I got the following problem:

I previewed a node (did not save it), and the uploaded image is now stored in the folder (with the correct path I specified with filefield_paths); it's of course a temporary folder ("[type]" in my case). But ... it's still there after I cleared the cache and run cron ...

Shouldn't they get deleted automtically? (can't delete them manually, because got not the needed rights - would have to log into my shared webhosting account ...)