There's a hidden board that I've used to test uploads, and it's got messages with attachments up to 10MB (I've tested this in the past). If you notice when you click the Additional Options... link when posting, you'll see the message:
Allowed file types: txt, doc, pdf, jpg, jpeg, gif, png, bmp, asf, wma, wav, mp3, midi, aiff, au, tdt, tlo, trt, tpt, csv, tst, tch, ttb, trl, xml, xls, htm, html, rar, zip, pic, reg, swf
Maximum attachment size allowed: 20480 KB, per post: 20
So the attachment size is set correctly. I'm using Chrome at the moment, and when you include a file as part of a post, Chrome gives you a status on the upload in the status bar section of the window. For whatever reason, the upload status for me never goes past 1%. I'm guessing there's a server issue. I'll try again tomorrow and if it persists I'll contact the hosting service.