How do you make sure an .jpg url actually is an image like it says it is?
We don't. If the website responds with "image" in it's Content-type header to our HEAD request, we proxy it. If not, it will be displayed as a URL on which a user can click.
If it turns out to not be an image during rendering, the image won't be rendered but I would blame the website then since they gave us wrong information.
The assumption is that this will work in 99% of the cases and when we find edge cases, we can think about other solutions.
Content-typeheader to ourHEADrequest, we proxy it. If not, it will be displayed as a URL on which a user can click.