There’s a bit of a long story here. You can read the description of the PRs if you’re interested in learning more. Per
https://github.com/discourse/discourse/pull/8286
and
https://github.com/discourse/discourse/pull/8291
If you’re on the latest, you should now be able to change the text that shows in the overlay in the editor
When you upload an image, you’ll get this

You can edit fileName to whatever you want the image overlay to show.
A quick note:
This was always possible but would have required you to know how markdown handles titles
The way to add a title in markdown looks like this

Given that not many people know about this, the first PR above adds the image alt as a fallback before defaulting to the file name. It being a fallback means that if a title is there, then it’s respected, if not, we use the alt.