So presumably you're posting a file that's over 1 MB, since that's the default max request size of nginx. We could bump that up, but I'd rather it be hosted somewhere else (Dropbox, Megaupload, etc.). If you'd like us to host it, let me or Abyssknight know and we can upload it to a directory on the server to be shared.
Currently the forum is set to allow individual attachment sizes up to 512kB and unlimited total size per post, so you should be able to upload reasonably sized pics in your posts. That's the main intent of allowing attachments.
Presumably the attachment shows because it creates the reference in the database before processing the attachment itself. We can always fix that.