When you make a post there is a area under the box you type in that says "Attachments", browse for the file you want and click add attachment, then it will upload the file and appear under your post.

I wouldn't suggest attaching big files though, because each user has a limited amount of space for their attachments.  So if its kinda big you can just upload it to Sendspace or Megaupload or a similar site, then post the link.