Some programs can create HTML pages that display MP4 files in the portal. These files may not play correctly due to the MP4 mime-type being missing from the portal website.
To fix this, go into the portal website and open up IIS Manager.
Click on the Site for the portal.
Double-Click on MIME Types under IIS in the Features View.
Check for the Extension .MP4. It probably will not be there.
Right-Click in empty space and click Add...
Make the window look like below:
You should now be able to view the MP4s embedded into the portal.