I've been searching for a couple of days now so I'll keep my question as short as possible to get out of your ways.
What I'm trying to do should be very basic with jQuery (append) I guess but for some strange reason I can't get it done while adding .js to the backend. Also this custom hack is just temporary, until the Gutenberg editor releases into the core and all this will not be needed anymore.
Simply add a "Hello world!" string to the Image Details modal where the red text on the screenshot(s) below is.
Screenshots below is a quick summit from what I'm doing.
This Image Details modal pops up when you press Edit image on a single image that is not part of your Media Library and doesn't have a Post_ID. Placeholder (URL) images for example.
Screenshot nr. 2. This Image Details modal is what you get when you click Edit image on a image that is added to a post the Media Library before. The beautiful Replace button below the preview thumbnail is poking my eyes, I want him, he's my Chuck Norris!