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