Custom apps load STLs as Segmentation by default

I have observed this is multiple custom app. When loading STLs in the Add Data dialog, the default option is as a segmentation.

Example affected app (AevaSlicer): https://data.kitware.com/#collection/5e387f9caf2e2eed3562242e/folder/5e3888e3af2e2eed35628e50

Start with the Add Data dialog in the custom application template and reproduce the STL-loading behavior using the linked AevaSlicer example. Trace why STL files default to Segmentation, then verify that loading an STL selects the intended non-segmentation option by default.

Written by the indexing model from the issue text.