Extract constant to configure valid mime types

This way it'll be possible to overwrite the valid mime types in a custom
model.

app/models/site_customization/image.rb

@@ -11,10 +11,12 @@ class SiteCustomization::Image < ApplicationRecord
     "logo_email" => [400, 80]
   }.freeze
 
+  VALID_MIME_TYPES = %w[image/jpeg image/png].freeze
+
   has_attachment :image
 
   validates :name, presence: true, uniqueness: true, inclusion: { in: ->(*) { VALID_IMAGES.keys }}
-  validates :image, file_content_type: { allow: ["image/png", "image/jpeg"], if: -> { image.attached? }}
+  validates :image, file_content_type: { allow: ->(*) { VALID_MIME_TYPES }, if: -> { image.attached? }}
   validate :check_image
 
   def self.all_images

spec/models/site_customization/image_spec.rb

@@ -44,4 +44,18 @@ describe SiteCustomization::Image do
       expect(map).not_to be_valid
     end
   end
+
+  it "dynamically validates the valid mime types" do
+    stub_const("#{SiteCustomization::Image}::VALID_MIME_TYPES", ["image/gif"])
+
+    gif = build(:site_customization_image,
+                name: "logo_header",
+                image: fixture_file_upload("logo_header.gif"))
+    expect(gif).to be_valid
+
+    png = build(:site_customization_image,
+                name: "logo_header",
+                image: fixture_file_upload("logo_header.png"))
+    expect(png).not_to be_valid
+  end
 end