Help Me To Understand The Waffle Image Library.

I have been ripping my hair out trying to understand what is going on in my app. I have managed to upload images through a form on the client side, then by using ecto-waffle I send the original %Plug.Upload to my AWS-S3 bucket. However, I am trying to resize the images uploaded BEFORE sending them to AWS. When I run diagnostic tests, it seems that my images are transformed every time I request them, however I want them to be transformed before this, and never again. It seems to me that the library doesn’t implement this behavior. I’ve searched through the docs for a few days now and still haven’t figured it out. If anyone knows how to resize a %Plug.Upload before calling cast_attachments(), I would be very grateful!