I went to the wrong place.
In setting, I don't see how to delete the files.
On my screen when I pull this up I have a little check box at the end of the top attachment, like the ones in PMs and subscribed threads.
If I check it, I can go to the bottom and I have a delete button opposite the " show thumbnails". If I hit, it the attachment will delete and the check box will move down to the next one.
The problem is that that you have to delete in order.
I added this glitch to George's "improve forum requests" thread.
I noticed that the check box and delete button are missing in your picture.
What is your OS and what version? Browser?