No snark intended: The same way you do elsewhere.
If you need to make multiple attachments from a single folder, once you've activated the Attachments button select the file or files from that folder in the same way you do in any other Windows program in the file selection dialog.
If it's from multiple folders, you will have to activate the attachments button and navigate to each folder successively, selecting the file or files you wish from each.
If there's some other specific problem please elaborate.
Brian - Windows 10 Pro, 64-Bit, Version 1903, Build 18362
Many of the insights of the saint stem from his experience as a sinner.
~ Eric Hoffer