The bug is more related to editing a post that had a download previously with a new upload
What i did:
1. Had a post with a demo map
2. Updated the demo
3. Deleted the demo from my post
4. Attempted to upload new demo
I've played around with attachments in the World Editor Zone for about 5 minutes now and I'm unable to reproduce this issue. I've tried uploading something, then deleting it, then reuploading it, then uploading different things to the same post, and everything works fine for me.
Have you tried using another computer (preferably on a different internet connection) to see if you still get the same issue? Can you send me the file you're using to see if it's something specific to the file that is causing the problem?