You can't just "upload" any video from your PC into any email in Prospect Reach.
But that doesn't mean that you can't use videos in your emails at all.
To be able to embed videos in your Prospect Reach meals, first, you must upload the video to any video hosting site such as Youtube, or Vimeo.
Note: Make sure that the video is set to Public.
Once you've uploaded your video, you can then grab its embed code and paste it into your emails in the Source code view.
Here's a video showing how it's done with Youtube and Vimeo videos: https://www.screencast.com/t/0D3H2tFSwrCl