Add setting to disable hotlinking of large image files の議論に続きます：
拡張子ではなく、MIME タイプによる画像の検出を提案したいと考えています。
ユースケース：
行に JPG の URL を置きます：
ファイルには JPG 拡張子がありませんが、content-type が正しく送信されているため、Discourse がこれを自動的に img タグに更新し、インライン表示し、さらにホットリンクすることを望んでいます。
現在のところ拡張子から行われているワンボックス化（img タグへの更新）のための写真検出を、MIME タイプから行うことが有益だと考えます。
Easy to work around though just add ?x=.jpg to the end of any URL.
https://freecannabis.com/media/dscf1924.36/full?x=.jpg
Thanks. But that doesn’t quite solve the problem. I’m already using workaround to add img tag manually. My users are not technically skilled so automatic form of hotlinking would be great.
I think it is super bad form to have images that look like HTML web pages so I would object to this being supported as it encourages super bad web hosting habits.
I agree it is bad practice of the image hoster, but you know - " be tolerant in what you accept "
And second, we already are pinging the URL, so why not use it to our benefit?
[edit]
Funny side note: I’ve just found that image hoster is using Xenforo Media Gallery paid plugin 
I’m going to disagree with you here. When you get the “Content-Type:” header, that’s when you know what you are dealing with. Otherwise a URL without an extension doesn’t look like anything, not “looks like HTML”.
This is how content negotiation on the web is supposed to work. I should be able to make a directory full of files, and return different versions of them depending on the “Accept” headers the browser offers. If you don’t explicitly say “Accept: image/x-mycoolformat”, but just a “Accept: /”, then maybe I want to hand you image/jpeg, but otherwise give you mycoolformat.
By that logic, page titles and urls also don’t matter. But we all know they do. So I violently disagree on this point.
Page titles have many uses, eg distinguishing tabs and titling bookmarks. URLs matter only because Google thinks they should. But I won’t press the point further.
