For accessibility reasons, alt text should be included for any images that are not purely decorative. The WikiEditor insert file dialog should include this option.

The equivalent VisualEditor dialog already has a field for alt text, and a good explanation message (visualeditor-dialog-media-alttext-section-help) that could be used:

You can use this to write a text description for people who can't see the item. The description should be enough for them to understand the purpose and information given by the media item. This is vital for blind users and other people using screen-reader software or text-only browsers.

Of course another way to fix this would be T209059: Replace WikiEditor widgets with OOUI-based (VisualEditor) components but I don't know if that might be contentious.