EDIT: what you're looking for is UTType.heic. Finally got Xcode beta up & running and was able to look at NSAdaptiveImageGlyph.contentType.identifier from the debugger and see that it's "public.heic" (UTType.heic)
I was looking at this: https://developer.apple.com/documentation/uikit/nsadaptiveimageglyph/4395169-contenttype.
NSAdaptiveImageGlyph will have a class var contentType which is a UTType, which you should take a look at. It's a convenience class provided by apple to reason about the type of data (no need to hard code "public.image", instead you can use UTType.image). AFAICT, there's no new UTType for adaptiveImageGlyph / Genmoji, etc. that's been added yet.
Maybe you could try UTType.heic The documentation states:
Adaptive images are compatible with the HEIC format, but include extra metadata about the supported resolutions and sizes.
This might unblock you in the meantime. However it sounds like NSAdaptiveImageGlyph will essentially be an HEIC with some additional data, so it may still get its own UTType later on.
HalfTimesTwo
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How did you create NSAdaptiveImageGlyph objects without Genmoji by the way? Also, I tried constructing a NSAdaptiveImageGlyph object from HEIC content in the pasteboard but was not able to do so. It's unclear as to wether Apple will be extending HEIC with their additional metadata or if HEIC will just be a way to access the raw image data only from an NSAdaptiveImageGlyph. I believe it will be the latter.