As an editor using VE within MediaWiki, I would like to be able to change the presentation form of an image (in this case from "thumbnail" to "fullsize") without loosing the "alt" text. I can provide the following example edit from OpenStreetMap wiki: https://wiki.openstreetmap.org/w/index.php?title=Sandbox&type=revision&diff=1791566&oldid=1791564. It looks to me like a data corruption, because the original "alt" data is lost but there would be no need to change it. The HTML contains the filename as alt text for the image.
I would expect:
[[File:Openstreetmap.jpg|alt=This is some OpenStreetMap logo including the label "OpenStreetMap".]]
But I get: