It’s actually a bug in Discourse. It used to work properly, until it didn’t.
We are not a hosting site, so we don’t allow you to embed pictures, videos, etc. in threads. You have to host the pictures, videos, etc. elsewhere and link to them.
At some point, Discourse managed to futz itself up and started treating the OneBox preview THAT DISCOURSE ITSELF MAKES as a user-posted image or video. So that’s why there’s a race. If you can post before it makes the preview, then it won’t block you. But if you are too slow, then it does block you.
There is a lengthy discussion in Site Feedback here:
We have asked the Discourse folks to fix it. So far, they haven’t.