To add documents/media to a card press the upload media button in the top right corner.
Then either drag the media to upload, choose files to upload, or choose files within Lumi (ie documents or media that have already been uploaded to Lumi) to link to the card.
You can choose documents, spreadsheets, presentations, images and videos.
You can find your attached media on the left of the card in the media section.
Please note: All images and documents can live on multiple cards at the same time, so if you update them, they update everywhere.... so no need to duplicate/re-upload them. You can edit these documents LIVE and AT THE SAME TIME as anyone else.