How does website detemine what's the full url when accessing resources like "/file.png"

I have a resource in my code that looks like that:

<img src="/upload/file.jpg">

when I open the resource, it looks like "http://ift.tt/2zy0CDv"

how does the website determine that the "base url" is "myapp.dev/en" and not "myapp.dev"? the file is accessible on "http://ift.tt/2nguAug" and i can hardcode it, but I want the "proper" solution.

if that helps I work with laravel