Thanks to both of you, I’ll try my luck on the forum there.
I am not sure if I should tick the “resolved” tick-box though. In doubt I don’t. Feel free to do it if you think it’s necessary. I’ll keep following replies here, just in case something new happens.
And in case anyone is interresting, here the topic I opened on the wordpress.com forums : https://en.forums.wordpress.com/topic/pre-fill-description-title-alt-text-and-caption-of-a-media
