Open WebUI has unauthorized deletion of knowledge files

Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to version 0.8.6, an access control check is missing when deleting a file from a knowledge base. The only check being done is that the user has write access to the knowledge base (or is admin), but NOT that the file actually belongs to this knowledge base. It is thus possible to delete arbitrary files from arbitrary knowledge bases (as long as one knows the file id). Version 0.8.6 patches the issue.