[Mac, Retina] Image loading for @2x images may try to append @2x suffix to wrong part of URL

Under rare circumstances the code to append @2x to an image filename may append it to the wrong part of the URL/filename. The code tries to find the image extension using lastIndexOf('.'), but if the actual image filename does not have an extension then it may find a period in an earlier component of the pathname and append the @2x before the period that exists there.