You can preview the video you just uploaded on the right side, in the Preview panel.

Here you can see the video file, exactly as it will be displayed in the post/tweet. You can click on Play button to play the video.

Please note that the preview might not work on all browsers and for all video file formats - we recommend to use Chrome browser and upload widely known video file formats (.mov, .mp4, .mpeg, .avi ... etc.)