Great question! If you are using the manual sync to create cards, you would create a `resources/` folder in your directory. From there, you can reference the attachment in the card's content file (see the Referencing Other Cards, Boards, Board Groups, and Resources section of the Manual Sync documentation)
If you are just trying to create a card via a `POST https://api.getguru.com/api/v1/cards/extended`, it is a bit tricky since we also use third-party file uploaders to accomplish this. You can to link to where the attachment is already being stored. So the only way to upload attachments to cards is via the sync process or the import process.
I will send this to product feedback to add an endpoint solely for attachment uploads so it is easier to reference attachments when creating cards via the API