Hey Max, it used to be just Vines and uploaded images that were expanded. With promotional tweets and the addition of open player cards in the new profile design, we should definitely add clearer documentation. I’ll work with Andy on this.
In our getting started guide, we discuss rendering priorities. Uploaded images will always trump the rendering of a card. However, the card meta tags are still useful for when other users tweet your content.
Let us know if you have any more questions about pre-expanded cards or rendering!