Windows Phone 8.1 Update 2 aka GDR2 is slowly making it out the gate to various phones.
The update comes preinstalled on the new Lumia 640 and Lumia 640 XL.
Enabling this homebrew makes the PSP an open platform.
The new patch fixes error C1-2858-3, which many users encountered due to this original bug.
In particular, if you downloaded Adrenaline yesterday, it is possible you ran into this issue. You can download by following the links in the official Readme here.
Windows asks if you want to convert the file, to which we answered 'no' on both occasions. On the device with Update 2, the video and thumbnail preview are now listed in the Xbox Video app, whereas nothing is shown on the device with Update 1.
Of course, Microsoft has already added native MKV support to Windows 10 for both PC and mobile (phone), so even if you do not get Update 2 – which is looking likely – you will get it with Windows 10.