To add my custom filetype I can use
Protected Overrides Function AddFileTypes(options As Rhino.FileIO.FileWriteOptions) As Rhino.PlugIns.FileTypeList
Dim result = New Rhino.PlugIns.FileTypeList()
result.AddFileType("LutraCAD (*.lcad)", "lcad")
Return result
End Function
Is it possible to disable all other options like STL etc?