Convert base64 string format for video file

I am trying to convert media file(video) from device video gallery to base64.
Because parse Files only takes base64.

//assume _videoURI is media file
var _videoURI = "this is media file"

//I tried below code but it doesn't work
var base64Video = "data:video/mov;base64," + _videoURI;

Does anyone convert video file successfully?