Shrine uploader and format conversion

I am trying to convert uploaded files to Web-friendly formats using the Shrine uploader. I've followed the examples from both Shrine and the ImageProcessing spin-off project to create this:

require "image_processing/mini_magick"

class ImageUploader < Shrine   include ImageProcessing::MiniMagick   plugin :processing   plugin :versions      process(:store) do |io|       original = io.download

      converted = convert(original, "png")

      {original: original, png: converted}            end    end

I'm using Rails 5.0.1, I've updated ImageMagick through Homebrew, When I upload a PSD file, I get the following info back from the upload:

:original: !ruby/object:ImageUploader::UploadedFile   data:     id: 43848eddee1b26135122fea3f138a628.psd     storage: store     metadata:       filename: shrine20161215-9079-1efvqf3.psd       size: 2881972       mime_type: image/vnd.adobe.photoshop       width: 600       height: 788 :png: !ruby/object:ImageUploader::UploadedFile   data:     id: d44ea65c825a6fa1085bf7a319999ed4.png     storage: store     metadata:       filename: mini_magick20161215-9079-hpru9s.png       size: 0       mime_type: 'cannot open: No such file or directory'       width:       height:

Has anyone used Shrine to make a format conversion like this?

Thanks,

Walter