It should be the /blueprints/files
folder.
You have to define the template to be used in your files section, where you upload the files.
For an example, check out the pages/album.yml
blueprint; this is the images
section from that blueprint:
sections:
images:
type: files
layout: cards
template: image
info: "{{ file.dimensions }}"
image:
ratio: 5/4
cover: true
min: 1
size: small
It uses the template
option to set the image
files blueprint.
There is no way to “select” a blueprint on file upload (like you can when you create a new page).