Load image without extention in html

In https://source.unsplash.com/ they provide a url for placeholder image only with url but not with a extension and the html code for load the image is

<img src=”https://source.unsplash.com/user/erondu/1600×900″> .

How this is done (loading an image without the correct source)?

how to build the same kind of api?

Thanks

More Similer Site:
https://placeholder.com/ (<img src=”http://via.placeholder.com/350×150″>)

http://lorempixel.com/ ( <img src=”http://lorempixel.com/400/200″ /> )