With the beta version of PRESENTA, you cannot upload video files. No worries, there's an handy way to use them, though.
You can embed videos as HTML embed, by using well-known platforms such as Youtube and Vimeo.
They can be used as a repository of your videos. This is how we, the PRESENTA team, include videos in our presentations. Both Youtube and Vimeo allow to set videos as private or unlisted (meaning, they won't be found by other users but you can use their embed code).
Then, paste the code within PRESENTA editor:
Now you can preview them by clicking their slides:
If you try to embed any external resources with the embed code, you'd realized once you click on the media element (i.e. the Youtube video to trigger the play button) that you won't be able to navigate between slides anymore. This happens because the browser focus now belongs to the media object instead of PRESENTA window. To get back the focus you can move the mouse cursor to the right side of the presentation window and clicking the white border.
There will be this feature in the near future for those who don't want or can use such streaming platforms.