Unless I missed it, there is no clean/direct way in PowerApps to attach a file from the user's OneDrive for Business. Sure, if they're syncing OD4B to their machine, they can use the out-of-box browse behavior of the standard attachment control to get a file, but there may be cases where that won't work for the end user. For example, we have a number of folks using Chromebooks or Linux and don't have the sync option.
I have looked at the "OneDrive for Business" connection in PowerApps, but it only seems to allow you to connect to an Excel file. While I'm sure that has it's uses, a simple "Attach File From OneDrive for Business" function would be ideal.