Set a wiki link to a source file

I’ve created wiki pages with links to source files. After some GitLab upgrade, this doesn’t work anymore now. The link shows me a form where I can create the non-existing wiki page. The wiki code looks like this:

[FileName.cs](src/Path/To/FileName.cs)

What is the definitive, reliable and stable way to create a link to the repository from a wiki page?