Maybe your k850 only play mp4, 3gp etc video format, first you need to check that, you have to convert your FLV video to MP4, 3GP, and then transfer it to phone. If you don't know how to do that, take a look at this step by step guide, hope it helps you
http://www.flvsoft.com/flv_to_3gp/ http://www.flvsoft.com/flv_to_mp4/ Good luck!