Ok after talking with HunabKu, this is not something that breaks with Mono, but with .NET and Assuming the existing code was meant for "Mono"...
So this "Mono" Fix is probably not needed...
And I actually tried to reproduce HunabKu trouble and his example Mob is clearly showing on my setup with the "obj" filter !
So I'm pretty sure this is some platform oriented trouble, even if the directory filter is pretty lame
(it can match obj, OBJ, Obj, oBj, ObJ... you got the point...)
I'll try to see with HunabKu how he triggered this