The premise of this is very simple, to bodge-fix what is really a bizarre design choice.
Currently, videos that are served via discord are directly linked within the website to a cdn.discordapp.com domain, followed by their associated IDs.
rewriting the domain to use media.discordapp.net allows the files to be viewed within a browser, as the content-disposition header is not present on these requests. (It also changes the content-type to be more accurate in some cases, mp3 -> mpeg3)

Unsure how URLs are presented in the discord UI (in comparison to slack's support), because design wise it brings up the issue that we'd also replacing the contents or intents of messages (why I've recommended this to be an option). As it would make sense not to edit the message text of anything coming through, and only the URL being different when clicked(/copied)

Sample content (contains music):
https://cdn.discordapp.com/attachments/306698121956687872/639660486568312832/monk.mp4
https://media.discordapp.net/attachments/306698121956687872/639660486568312832/monk.mp4

Sorry, I am having a hard time understanding this. Could you rephrase it?

In short, having an option to rewrite incoming CDN discord URLs onto the alternative domain bypasses the download requirement of any video file, and plays in the user's browser.

It's a nifty QoL feature, but would it still be ethical to do even as an opt-in behaviour

Implemented in next version. Only works if the contents of the message as sent by the Discord server indicate that there is something at the media subdomain, which it usually does. (Ripcord won't blindly rewrite it.)