WhatsApp currently is kind of picky about the video format that supports. To my knowledge only: MP4 (H264 + aac)
I use avconv to convert my existing video files:
INPUT_FILE=~/vids/ground_spiral.MP4
avconv -i $INPUT_FILE \
-f mp4 -c:v h264 -b:v 1200k -g 300 -bf 2 -s 640x352 \
-c:a aac -b:a 64k -ac 1 \
/tmp/output.MP4