We don't yet have fully "on purpose" multimedia support on mobile -- eg if it works at all, that's awesome. :) But it's probably sub-ideal in a number of ways. (On iOS in particular we have *no* playback except in the desktop mode due to Safari's lack of native WebM or Ogg support; the ogv.js JavaScript playback has only been integrated on the desktop mode so far, as we need to clean up TimedMediaHandler's JS-side code to run cleanly on mobile... and not suck on desktop.)
Questions:
* Are you viewing the File: page in a browser directly, or some page that includes the file on it? (If the latter, which page?)
* Are you pressing the 'play' button on an image thumbnail, or clicking the "download original file" link, or something else?
* What device are you using?
* What Android version are you running on?
General issues:
* There's no manual resolution selection override in the user interface, so you might be getting a high resolution file that's too slow to decode.
* In Firefox in particular you may not be getting the benefit of hardware acceleration for WebM video decoding.
* The 'Android default browser' may or may not exist on any given device (many newer devices just have Chrome, so I can't test it locally on my Nexus 5 or 5x).
There may or may not be any 'fixes' we can make in a short term. Note there are *no* WMF resources assigned to video at present, so things get fixed only as someone interested in the topic gets to them.
-- brion