Handling image/x-png in Image.pm

I think this is the right fix for the underlying issue for #55235 in RT. According to the W3C, 'It is recommended that implementations also recognize the media type "image/x-png"' (http://www.w3.org/TR/PNG/#A-Media-type) so that's all I've done.

Let me know if there's anything I can do towards getting this merged/released :-)