EXIF inside of PNGs is new. You can make it work by embedding structured chunks into the file, but it's not official in any way (well, not until the new spec, at least). Sidecar files have some kind of interoperable format that at least don't break buggy PNG parsers when you open the image file. The sidecar files themselves differ in format, but at least they're usually formatted according to their extension.
The usual sidecar files, XMP files, are standardised (in that they follow a certain extensible XML structure) and can (and often do) include EXIF file information.
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EXIF inside of PNGs is new. You can make it work by embedding structured chunks into the file, but it's not official in any way (well, not until the new spec, at least). Sidecar files have some kind of interoperable format that at least don't break buggy PNG parsers when you open the image file. The sidecar files themselves differ in format, but at least they're usually formatted according to their extension.
The usual sidecar files, XMP files, are standardised (in that they follow a certain extensible XML structure) and can (and often do) include EXIF file information.