I am writing my manual right now and there's coming a question in mind. If I attach an image to a post, publish the post, edit it then and delete the picture, the following is happening:
1. The admin can't see the image in his admin center anymore.
2. But: the image file seems to be kept on the server. It has the format *.elk and if you open it (for example with a browser), you will get css code! :o :D
My question is: is this file somehow encoded?
I did a couple of tests but the attachments seems to be properly deleted from the server (Even thumbnails).
The files are not encode (at least not from image to css, with some settings images maybe re-encoded to the same format for secutiry reasons, but the result would still be an image).
If you find css code maybe it is not the image, but a css you attached somewhere else?
Is it a test forum? It may be the result of a previous experiment?
The reason for the .elk extension was to "force" filezilla in to a binary format for those files, otherwise (depending on filezilla settings) it may transfer as ascii files and mess up the files. The filename is hashed so once uploaded its "hard" to find, thats for security.
I found the same as emanuele, in some quick testing on a test site, the files are being deleted as expected. Maybe a server permissions issue if its an older uploaded file from a backup?