def make_modifier_scale(scale):
    """Make a string designating a scaling transformation, typically for
    filenames of rescaled images.
    
    Args:
        scale (float): Scale to which the image was rescaled. Any decimal
            point will be replaced with "pt" to avoid confusion with
            path extensions.
    
    Returns:
        str: String designating the scaling transformation.
    """
    mod = "scale{}".format(scale)
    return mod.replace(".", "pt")