Windows 8.1 natively supports mkv files
Microsoft first started supporting the MKV file format natively on the company’s Xbox One console earlier this year, and now the company is bringing those changes to Windows. Starting today, Windows 8.1 will now natively support the Matroska Multimedia Container (MKV) file format with the built-in video app. The open standard container format has long been used to provide pirated copies of movies and TV shows through BitTorrent or other file sharing sites, but Microsoft’s move to provide native support lends the file format some much-needed legitimacy.
> While it’s likely most content providers will continue to provide streaming video instead of DRM-free download options, native MKV support in Windows adds another option to share video or audio files without having to download third-party players like VLC. Microsoft has also pledged to support MKV in the upcoming release of Windows 10, alongside support for Free Lossless Audio Codec (FLAC) files. Microsoft’s MKV implementation in Windows 8.1 is still limited by the operating system's codec and subtitle support, but the company may choose to improve both of these drawbacks in Windows 10.
Source: TheVerge