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