Oh, you have a native trigger that creates the item.
In that case, as a workaround, you could add a webhook (or another recipe with a native trigger and custom action) that notifies you when an item is created so you can get the ID (you can think of a way to filter items created manually vs the ones created with the recipe by maybe adding a checkbox, or looking for a specific status or something like that). Then, when you get the webhook, you check that, and using the column ID from the recipe, you can add the file.
Yes, I understand. It is a workaround but its a bit cumbersome. I think I will wait for hopefully the dev team to add file uploads to dynamic mapping. Is there a link to the feature request where I can monitor the progress ?