Called during a PUT request where the action specifies
        a copy operation. Returns resource URI of the new file.

def _copy(self):
        """
        Called during a PUT request where the action specifies
        a copy operation. Returns resource URI of the new file.
        """
        copypath = self.action['copypath']
        try:
            self.fs.copy(self.fp,copypath)
        except OSError:
            raise tornado.web.HTTPError(400)
        return copypath