Files are not copied to child jobs workspace if it was uploaded by File Parameter.
This functionality was working few months ago. It is probably connected with security-170
How to reproduce:
1) Create matrix job with few axis
2) Make job parametrized with at least one File Parameter
3) As a build step print content of WORKSPACE (dir or ls)
4) Run job and upload some file.
5) WORKSPACE of child jobs is still empty.