Out of curiosity, anyone know if the file explorer in Windows 8 handles flac tags ?
Yeah! Thank you Tim and Seth! Thanks for the link! I will upload that thing to mediafire, cause it has to last looong.
FLACPH15.rar
So you're basing the presence of FLAC support on just the Year and Publisher tag? I suggest that you check other tags like Length, Artist and Title. Native FLAC support has been added in Windows 10; that's a fact. If *you* would Google around a bit, you'd quickly find that out yourself.
And yes, certain FLAC tags aren't recognized, but that's because Microsoft's support isn't complete yet. I suggest that you use the feedback tool in Windows 10 to report that.
I'm not going to further comment on M4A as that's off-topic here.

Apologies for the late reply, I was meaning to reply sooner but lost sight of your post.
There's actually a predefined list of properties that Windows supports, it's documented here. While it's possible to define custom properties (explained here), doing so requires work from the developer of the particular Property Handler providing the custom properties. It's unfortunately not possible for individual users to define their own custom properties.