twisted.web: returning dynamically generated images as image/png

I have the following setup (see below), in which the render method of
a RasterServer instance is responsible for generating a png file data
in memory that then should be passed on to the client that made the
request.
However, I can't get my head around how to define the content-type for
the render method result? Or whether that's possible at all? If not,
how should I define my RasterServer so that it would return image/png
content to the client?

Cheers,
Jussi

========================================

from twisted.web import resource, static, server
from twisted.application import service, strports
...
root = resource.Resource()
...
raster = RasterServer()
root.putChild('wms', raster)
...
application = service.Application('test')
mo_server = strports.service(where, server.Site(root))
mo_server.setServiceParent(application)
----------------
class RasterServer(resource.Resource):
   def render(self, request):
       ....