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That's just concatenating the files together and relying on the zip extractor to ignore the PNG part of the file, rather different than actually embedding it in the image data itself. I remember this being a popular way to upload non-image files to certain imageboard websites a few years back, to get around them only allowing image uploads. JPG+RAR was the usual choice.

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The data was visible as a block of jumbled pixels as the bottom, not sure how concatenated files render visually (especially jpg format), so for all I know it may have been exactly what you're describing. Pretty sure I found it on an image board anyway.

That's how it looks when you concatenate two file types. I guess in theory if you make the image a pixel, you effectively get the same output but with way less effort.

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