The Issue seems to start with ITMS 8.5 RU4, but is reproducible through 8.6 RU1. You are able to upload the documents by typing in the path. The Notification Server is not listed under Network as it was in 8.5 RU3 or earlier versions.
Steps to reproduce:
In 8.5 RU3 you see this:
In later versions, you see this (no Notification Server and local drives):
By design due to the security limitations of various browsers.
Asset Management Solution/Suite 8.5 RU4 through 8.6 RU1
The documented method is to type a UNC path into the dialog box labeled Path and clicking OK.