Increase preview card image size limit from 2MB to 8MB when using libvips (#31904)

spec/models/preview_card_spec.rb

@@ -3,6 +3,12 @@
 require 'rails_helper'
 
 RSpec.describe PreviewCard do
+  describe 'file size limit', :attachment_processing do
+    it 'is set differently whether vips is enabled or not' do
+      expect(described_class::LIMIT).to eq(Rails.configuration.x.use_vips ? 8.megabytes : 2.megabytes)
+    end
+  end
+
   describe 'validations' do
     describe 'urls' do
       it 'allows http schemes' do