theres a manual fix for this. in mbin, when you attach the image in a post and save, it is uploaded and shown. you can then grab the image link (right click image, copy link), edit the post, and add the link using the image tag, and save, it then shows in-line in lemmy.
yeah, it sucks but at least it works until this is implemented. ive been usin this method for over a year now..
This PR doesn't add that, but anecdotally I'm fairly sure it already works for Mastodon... Not a Mastodon user, so I can't find any examples though. They definitely show on Mbin tho.
I just tried with Masto - maybe there's different versions, but it didn't work with the one I tried.
Screenshot:
It's probably for the best that this PR doesn't also convert inline Markdown into an attachment to send out for Mastodon's benefit, because then there would be the danger of apps that understand both showing two images. It'd be better if Mastodon did the translation when receiving stuff, but Mastodon doesn't seem as good as MBIN when it comes to co-operating with Lemmy.
(edit: how that screenshot shows on MBIN is a bit disappointing though - at least looking at on the web)