fix appearance of menu File-->Open

[dave-doty]

It should look more like Save, but note how it appears to have some padding around it that Save does not have:

Here's the HTML you see for each of them if you right-click and select "inspect":

But since they are different React Bootstrap components (FormFile in the first case, DropdownItem in the second case), I'm not sure how to get FormFile to have the same behavior as DropdownItem.