you can check the type of video that you have downloaded/downloading..just look at the file's name..
if you are downloading an avi video, the file should be smtg like 'video.avi' while an mp4 video would be 'video.mp4'..
most players are not mp4 compatible..the only way to change this is to re-encode the video which is pointless as most of the time, there are avi files available..just check what file you are downloading 1st before downloading..good luck!!
