My account tools menu (for investment accounts) is the same as shown by splasher (with no selection to attach files to transactions) and my preferences are the same as shown by Rocket J Squirrel (with "Attach button" selected). I, too, don't see any way add attachments to investment transactions. I'm using Quicken Premier 2017 for Windows. This behavior doesn't match what the help file and support website indicate, and it was a useful feature in Quicken 2015. This has got to be a bug.
Richard,I'm the guy who wrote "Omigosh, I see it now!!!!," so don't take what I say below as a criticism.When the investment register window opens, there is no button apparent for attaching files to transactions, whether you are in the register pop-up mode or not. It just isn't there. Don't try to find it in the gear (action) button drop-down list at the top right of the window, or in the window you get by clicking the "Enter transactions" button at the top left. It isn't in either of those places, either. (I think you were looking in the latter.)
The way to make the attach button appear is to click on a transaction in the register list. Then the button appears in the second line of the transaction, in the rightmost column. You may have to widen the window or scroll to the right to see it.
Does this help?