I'd have been tempted to rename the file to foo.jpg.png and rely on the fact they'll have "hide known extensions" on. Or even just rename to foo.jpg and figure

I'd have been tempted to rename the file to foo.jpg.png and rely on the fact they'll have "hide known extensions" on. Or even just rename to foo.jpg and figure the MIME type and file header would be enough for browsers to still render it like a PNG.