Opera is capable of playing h.264 video in an mkv container for example. But, Opera doesn't have a video/x-matroska mime-type/file-type handler to embed the file with the HTML VIDEO element when you navigate directly to the file. Therefore, the file is downloaded instead. Same in other browsers.
Note this issue just started in last 2 days....
Think we need a link to an mkv file that worked a few days ago for you but doesn't now.