> Currently we have to hide the file name if an image preview is available. Should that be different for videos, really?

yep, see https://mozilla.app.box.com/s/wzgsb3lkqglv0dnfdgzs/1/864518456/7994808358/1 page 36

> (if yes, would that be an issue to add the file names when we really need them?)

I'd prefer to play with css classes and always adding filename for accessibility reason