Admin can change the format that Spree::Images are stored/served in

app/controllers/spree/admin/image_settings_controller_decorator.rb

@@ -0,0 +1,11 @@
+Spree::Admin::ImageSettingsController.class_eval do
+  # Spree stores attachent definitions in JSON. This converts the style name and format to
+  # strings. However, when paperclip encounters these, it doesn't recognise the format.
+  # Here we solve that problem by converting format and style name to symbols.
+  def update_paperclip_settings_with_format_styles
+    update_paperclip_settings_without_format_styles
+    Spree::Image.reformat_styles
+  end
+
+  alias_method_chain :update_paperclip_settings, :format_styles
+end

app/helpers/admin/image_settings_helper.rb

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+module Admin
+  module ImageSettingsHelper
+    def admin_image_settings_format_options
+      [['Unchanged', ''], ['PNG', 'png'], ['JPEG', 'jpg']]
+    end
+
+    def admin_image_settings_geometry_from_style(style)
+      geometry, format = admin_image_settings_split_style style
+      geometry
+    end
+
+    def admin_image_settings_format_from_style(style)
+      geometry, format = admin_image_settings_split_style style
+      format
+    end
+
+    def admin_image_settings_split_style(style)
+      [style, nil].flatten[0..1]
+    end
+  end
+end

app/models/spree/image_decorator.rb

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+Spree::Image.class_eval do
+  # Spree stores attachent definitions in JSON. This converts the style name and format to
+  # strings. However, when paperclip encounters these, it doesn't recognise the format.
+  # Here we solve that problem by converting format and style name to symbols.
+  # See also: ImageSettingsController decorator.
+  #
+  # eg. {'mini' => ['48x48>', 'png']} is converted to {mini: ['48x48>', :png]}
+  def self.format_styles(styles)
+    styles_a = styles.map do |name, style|
+      style[1] = style[1].to_sym if style.is_a? Array
+      [name.to_sym, style]
+    end
+
+    Hash[styles_a]
+  end
+
+  def self.reformat_styles
+    Spree::Image.attachment_definitions[:attachment][:styles] =
+      format_styles(Spree::Image.attachment_definitions[:attachment][:styles])
+  end
+
+  reformat_styles
+end

app/overrides/spree/admin/image_settings/edit/add_image_format.html.haml.deface

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+/ replace_contents '#styles_list'
+
+- @styles.each_with_index do |(style_name, style_value), index|
+  .field.three.columns
+    = label_tag "attachment_styles[#{style_name}]", style_name
+    %a.destroy_style.with-tip{:alt => t(:destroy), :href => "#", :title => t(:destroy)}
+      %i.icon-trash
+    = text_field_tag "attachment_styles[#{style_name}][]", admin_image_settings_geometry_from_style(style_value), :class => 'fullwidth'
+    %br/
+    - current_format = admin_image_settings_format_from_style(style_value) || ''
+    = select_tag "attachment_styles[#{style_name}][]", options_for_select(admin_image_settings_format_options, current_format), :class => 'fullwidth', :id => "attachment_styles_format_#{style_name}"

spec/features/admin/image_settings_spec.rb

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+require 'spec_helper'
+
+feature %q{
+    As an admin
+    I want to manage image formats
+} do
+  include AuthenticationWorkflow
+  include WebHelper
+
+  before(:all) do
+    styles = {"mini"    => "48x48>",
+              "small"   => "100x100>",
+              "product" => "240x240>",
+              "large"   => "600x600>"}
+
+    Spree::Config[:attachment_styles] = ActiveSupport::JSON.encode(styles)
+    Spree::Image.attachment_definitions[:attachment][:styles] = ActiveSupport::JSON.decode(Spree::Config[:attachment_styles])
+    Spree::Image.reformat_styles
+  end
+
+  scenario "setting the image format for a paperclip style" do
+    # When I go to the image settings page
+    login_to_admin_section
+    visit spree.edit_admin_image_settings_path
+
+    # All the styles should default to "Unchanged"
+    page.should have_select 'attachment_styles_format_mini',    selected: 'Unchanged'
+    page.should have_select 'attachment_styles_format_small',   selected: 'Unchanged'
+    page.should have_select 'attachment_styles_format_product', selected: 'Unchanged'
+    page.should have_select 'attachment_styles_format_large',   selected: 'Unchanged'
+
+    # When I change a style to "PNG" and save
+    select 'PNG', from: 'attachment_styles_format_mini'
+    click_button 'Update'
+
+    # Then the change should be saved to the image formats
+    page.should have_content "Image Settings successfully updated."
+    page.should have_select 'attachment_styles_format_mini', selected: 'PNG'
+
+    styles = Spree::Image.attachment_definitions[:attachment][:styles]
+    styles[:mini].should == ['48x48>', :png]
+  end
+end

spec/models/spree/image_spec.rb

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+require 'spec_helper'
+
+module Spree
+  describe Image do
+    describe "attachment definitions" do
+      let(:name_str)  { {"mini" => "48x48>"} }
+      let(:formatted) { {mini: ["48x48>", "png"]} }
+
+      it "converts style names to symbols" do
+        Image.format_styles(name_str).should == {:mini => "48x48>"}
+      end
+
+      it "converts formats to symbols" do
+        Image.format_styles(formatted).should == {:mini => ["48x48>", :png]}
+      end
+    end
+  end
+end