An STL Midplane mesh is not a standard format. However, if you need to import a Midplane mesh that is stored in STL format, follow these steps:
Import the STL. In the Import dialog, set the model type to Dual Domain, not Midplane, and then click OK.
Click (Mesh tab > Mesh panel > Generate Mesh) and create a mesh on your model. This process will remesh the STL surface into a Midplane mesh (but the type will still be set to Dual Domain).
When the Mesh complete! dialog appears, click OK.
In the Study Tasks pane, right-click Dual Domain Mesh to change the mesh type to Midplane.
Assign properties (such as part surface) to the newly-created mesh.