You will need to install a user script manager extension to install this script.
I wanted my users to be able to download movies and music without having to buy a plex pass, This script adds a download button to the bottom of the media player controls allowing you to download movies and music without buying plex premium.
Simply put, It searches for a audio or video element on the page and grabs the current source, appending the download=1 parameter.
Transcoding downloads won't work since you're grabbing the original file directly, Just click "Original Quality" in the quality options and it will work fine.
Contact me if you have an issue with the script: http://ksir.pw