Using @ naming convention animations...?

I have an older asset that I am trying to use, and it used to work okay (back in Unity 4.x), but now I am unable to get animations working when they are in separate fbx files (using @animationName suffix) instead of all combined into one file.

Any ideas? I have tried creating a generic rig and putting them into an AnimatorController, but the default animation won't play...

So then I tried setting to Legacy, but I can't find a way to access the animations in Legacy mode when they are separate files...

Any ideas or documentation/tutorials for how to set this up?