1 As I recall, your phone isn't particularly new.  What version of Android does it run?  I know I had a real hard time getting my tablet to handle fansubs, and ended up just transcoding everything I wanted to watch on it.  Newer versions of Android have included more codecs and such, but your phone probably doesn't recognize MKV files.  Now, why that crashes windows explorer is a different matter...

2 The most recent patch upgraded us to 4.4.2.

3 But I can't see why the Android version would cause the Windows Explorer on my Win7 computer to shut down and restart.

4 Is there a registered extension to Explorer that is trying to open--or perhaps even *stream*--the MKV file?   Perhaps something you installed a few years ago on that box and have since forgotten or even "uninstalled" while leaving some cruft behind?

5

Wouldn't it also try to do the same for WMV, AVI, and MP4?

To directly answer your question: I don't have the slightest idea.

When I copied the AVI and MP4 files across, there was a popup asking me if I wanted to "convert" them first, because my device might not be able to play them. I said "no" and they played fine.

For the MKV, the crash happened at the point where that popup would have appeared, so I would guess that there's a problem in that recognition code.

It isn't very important; I don't really want to play video files on my phone. The reason I was doing it today is that it's been about a year since I've done a deep discharge of my phone's battery, and it was about time. (Lithium batteries don't have significant "memory effect" but it's still good for them to do a deep discharge once in a while.) So it claimed to be about 50% and at the rate it was using power it would have taken until Friday. I figured that playing a video was a good way to 1. Stop turning off after 90 seconds of no keyboard activity, and 2. use considerable amounts of power.

And it worked, though it took about three hours to reach 10%.