Get a thumbnail as string or None if the file isnt an image

        size would be one of JFSFile.BIGTHUMB, .MEDIUMTHUMB, .SMALLTHUMB or .XLTHUMB

def thumb(self, size=BIGTHUMB):
        '''Get a thumbnail as string or None if the file isnt an image

        size would be one of JFSFile.BIGTHUMB, .MEDIUMTHUMB, .SMALLTHUMB or .XLTHUMB'''
        if not self.is_image():
            return None
        if not size in (self.BIGTHUMB, self.MEDIUMTHUMB, self.SMALLTHUMB, self.XLTHUMB):
            raise JFSError('Invalid thumbnail size: %s for image %s' % (size, self.path))
        #return self.jfs.raw('%s?mode=thumb&ts=%s' % (self.path, size))
        return self.jfs.raw(url=self.path,
                            params={'mode':'thumb', 'ts':size})