It doesn’t currently appear possible to add alt text to images without using the SetAttribute widget. As the widget needs to be used inside a DataView, we need to rely on Javascript to add alt text on images outside of them, such as in Document templates.
I feel this should be a readily available option in the image properties, alongside ‘name’, ‘class’, and ‘style’.
This was first mentioned by Jeannie Versteeg 10 months ago:
https://forum.mendix.com/link/ideas/1323
I noticed the reference in the release notes for 8.6 is incorrect: https://docs.mendix.com/releasenotes/studio-pro/8.6#860
And make sure those texts are translatable.