After coming back to this topic which I created years ago, I’ve been able to look at this from a fresh perspective.
There’s a few corner cases like what @nonoesp posted which might create potential confusion for a syntax that originally file extension-independent. If there’s ever an image extension that clashes with an audio or video extension in the future (e.g. if someone introduces a .ogg, .mp4, or .webm for images) we’d have a problem.
Perhaps these issues are not enough to be blockers, but there’s also a nicer syntax that we could use for file extension based content blocks:
Because this syntax is intended to be file extension based from the start, there’s no question confusion about the existing behaviour of the Markdown image syntax - it’s a clean break from existing syntax that is also simpler for writers. It also improves how images are embedded by placing them inside figures. I’d be in favour of uniting around this syntax (as an extension to the main spec) for embedded content instead.