I wonder, is there a chance you could make an userscript out of this?

[TsukiZero]

just for convenience for users that may need to patch it themselves if Reddit does something.

[a-pav]

I think it will most likely introduce an infinite loop.
In a userscript, AFAIK, best option to use for this case is // @run-at document-start pragma, which is good for redirecting the HTML interface to the direct-image-url but upon loading that url — since the headers are set to NOT accept a direct image — it will get redirected back to the HTML interface once again; just like when you try to open the direct-image-url in a new tab.

Eidt: I don't think the more advanced GM_ options can give access to HTTP headers either. see this.

[themojache]

just for convenience for users that may need to patch it themselves if Reddit does something.

Here is my attempt: https://github.com/themojache/userscripts/blob/main/reddit-image-redirect.user.js.

• This script has some alterations, I find it works better for my purposes (/gallery/ endpoint support and old reddit gallery support mainly).
• Base64 DataURL's are not ideal (image file size expands to 4/3 of the original file size), but makes original images viewable without an extension. This script generates DataURL's on demand locally, after fetching the image in a similar manner.

Feel free to open an issue/PR for suggestions.