Hi @Zaff_Zaff
For STEP the extension should be either .stp or .step, for Parasolid it should be .x_t.
Have you tried using OnShape’s Academic Version instead of Siemens NX? I suggest this because, as I had mentioned, STL files are really not ideal for simulations in the platform (especially if you’re using solvers other than CHT (IBM), Multi-purpose, LBM or PWC).