This app is one of the better mobile device forum apps that will let you do it. Posting attachments through a mobile browser is a PITA otherwise (if it's even compatible). Yea, it's $4, but beats trying to use the browser.
On what Grady said, that is your best option for browsing the forum on ANY smart phone. The tapatalk app is well worth the money. And it works on thousands of other forums, not just this one.