Hello,
Perhaps this is a total newb question. I have setup Google Drive in my Trello board. I can select "Attach a File" and I get a popup window. There is nothing in that popup window and I can't therefore select any file. I am using the web version of Trello in Chrome on a Mac. I had issues setting up Google Drive due to my use of Privacy Badger, but have disabled Privacy Badger on Trello, so that should no longer be an issue.
Marc