Last modified: 2013-06-29 02:13:15 UTC
It looks like at least alt used to work at some point, but does not right now.
echo '[[File:Wiki.png|alt=foo|class=someclass]]' | node parse <body><p data-parsoid='{"dsr":[0,41,0,0]}'><span class="mw-default-size someclass" typeof="mw:Image" data-parsoid='{"optList":[{"ck":"alt","ak":"alt=foo"},{"ck":"class","ak":"class=someclass"}],"cacheKey":"[[File:Wiki.png|alt=foo|class=someclass]]","img":{"h":155,"w":135},"dsr":[0,41,null,null]}'><a href="./File:Wiki.png" data-parsoid='{"a":{"href":"./File:Wiki.png"}}'><img alt="foo" resource="./File:Wiki.png" src="//upload.wikimedia.org/wikipedia/en/b/bc/Wiki.png" height="155" width="135" data-parsoid='{"a":{"alt":"foo","resource":"./File:Wiki.png"},"sa":{"alt":"alt=foo","resource":"File:Wiki.png"}}'></a></span></p>