You can upload the video to youtube first from your phone. You need to youtube account. The mobile app on Youtube allows you to upload videos.
Once it is on Youtube, copy the link to the video you uploaded and past it in your steem blog. Steemit is more like reddit since it does not allow direct uploads like facebook and youtube, so you have to use links for pictures and video.
Ooooooh!!! So thats why?! Thanks for throwing some light here!