select the contact's Files page, then press the + button to bring up the file selection window. You can select multiple files and press the Import button
select the contact's Files page, then drag-and-drop files into the files section from Finder. You can also drag-and-drop emails from the Mail app
select the main Files section from the app's left sidebar, then select the + button; you will have to first select the contact, then select the files you want to import in the files selection window
select the main Files section from the app's left sidebar, then drag-and-drop files into any existing contact's files section. You can drag-and-drop files from Finder, or emails from the Mail app.
with the Files section selected from the app's left sidebar, you can drag-and-drop files and emails onto the "Files" button, and then select a contact that isn't visible or doesn't have an existing section in the Files page.
with either a contact's Files section selected, or the general all Files page selected, you can paste a file from the clipboard into Contacts Journal. You will be asked to select a contact
