That would generally only happen if it hasn’t finished uploading. It’s finished uploading when the text boxes go edit the title and description are visible.
Actually, it doesn't appear to work in a post on your forum either:
The underlying response from the server is a 403 forbidden error:
HTML:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>403 Forbidden</title>
</head><body>
<h1>Forbidden</h1>
<p>You don't have permission to access /forum/attachments/upload
on this server.<br />
</p>
</body></html>
This likely isn't an error coming from us - it seems like it's being thrown by the web server itself.