If the only media used in an article is an movie, the picture in the hovercard will still be black.
Topic on Talk:Page Previews
movies in hovercards
Thanks for the report.
If you happen to remember where it was, could you link to an example?
I've tried to test it at my meta sandbox (linked from my userpage there) and I cannot seem to get an image in the hovercard, for either .webm or .ogv movie files, in thumbnail or fullsize...?