Tornado handler thinks POST is missing argument when Firebug shows the argument being sent

I have a simple form using a POST method, consisting of a text box and a file. After hitting submit, I can see the post in Firebug as follows:

Parts       multipart/form-data
posttext    Some text
image   BlahJFIFBlahExifBlahPhotoshopBlahBinaryStuff etc...

The Tornado handler that receives it looks like:

class NewPostHandler(BaseHandler, MessageMixin):
    @tornado.web.authenticated 
    def post(self):
        message = {
            'posttext':self.get_argument('posttext'), 
            'image':self.get_argument('image'),
            etc          

But Tornado's handler returns:

[W 100618 23:07:32 we开发者_运维知识库b:775] 404 POST /a/message/new (127.0.0.1): Missing argument image

I'm not quite sure what I'm doing wrong here. Am I correct in thinking 'argument' means a input element's 'name' attribute? How can I make the handler see the argument?

Thanks for your help, I've been struggling with this for an hour and must admit I'm stumped!

---

For file uploads you should use self.request.files instead of self.get_argument().