If you are answering a question that has been posted by someone else, you can also click on the "Upload an Image" button that appears just above the "answer window."
If you are not uploading an image but want to upload a link to a file somewhere on the Internet, all you have to do is copy and paste it in your answer or in your original post.
If you want to upload a file like a Word document, I don't think you can do that. You should be able to do a copy and paste into your answer/question, though, if it isn't too long.
Blue's answer is your best bet, though. He's the inventor, gatekeeper, and grand poobah of this site. :-)
