Support on this site? Put one in an <img> tag (on any site) and you'll know. Then we can ask the powers that be to include the file type in the list. (That's a trivial change.) If it's going to take a major change to allow the show to display the format, it probably won't get done, unless it becomes a common format. (And I think it's the latter - HTML5 can't display an HEIF container, to the best of my knowledge.)
And, as Mooncatt said, it
is a container format, not a picture format. If you use a single .jpg picture in an HEIF container (don't know why you'd want to), or an mpeg4 (again, don't know why you'd want to), it may not be too much work. If you use some unknown (to us) picture or video format, the odds are that the site won't be modified to open the container, just so we can't display what's in it (like a series of jpg files).
If you read
Apple’s HEIF image format choice reinvents photography, you'll see that most of what HEIF containerization does isn't needed (or wanted) on a forum. Most of the things here are links to a YouTube video or are screenshots (which have no depth, sound or anything else) - about the only thing HEIF
might give us is a smaller file size.