To delete a FileChat, click on the FileChat setting icon (the small gear) above the file list on the right, choose "Delete this FileChat" option. You'll have to confirm you really want to delete the FileChat.
When you delete a FileChat, you do NOT delete the files in your cloud storage.
But all the chats and the likes will be gone, and there is no way to restore them :(
Remember, guests cannot delete a Filechat so you can chat safe.