Steps to reproduce:

1) Download attached project "Unity-Dll-Bug.zip" and open in Unity.
2) In Project tab, open Plugins folder and click on VideoInput.
3) Check Inspector tab and you will notice there is no checkbox for architecture platforms(see attached image "No.png").
4) Now go to this project location >> Assets >> Plugins >> And rename VideoInput.Dll in to VideoInput.dll(change capital D).
5) Go back to Unity and reimport VideoInput.dll file.
6) Check Inspector tab again, notice that you can choose between architecture platforms (see image attached "Yes.png")

Tested versions: 5.2.4f1, 5.3.5p4, 5.4.0b22.