Moving from plugin "Processing" to Derivatives. How to process original?

[james-em]

Hi,

We currently are processing plugin using https://github.com/shrinerb/shrine/blob/master/lib/shrine/plugins/processing.rb which allows us to process the original uploaded file.

Because it's now deprecated, we would like to move to the new recommended "derivatives" options, however, we couldn't find a way to process the original file.

Here is an example scenario. Let's say a user upload a 60MB image and we process it to our maximum accepted value i.e 5MB 2000x2000, we don't want to store this huge 60MB image, nor we will ever use it.

How can this be achieved?