Use mime type symbol to set content_type for custom css response (#37845)

app/controllers/custom_css_controller.rb

@@ -3,7 +3,7 @@
 class CustomCssController < ActionController::Base # rubocop:disable Rails/ApplicationController
   def show
     expires_in 1.month, public: true
-    render content_type: 'text/css'
+    render content_type: :css
   end
 
   private

spec/requests/custom_css_spec.rb

@@ -13,11 +13,10 @@ RSpec.describe 'Custom CSS' do
         expect(response)
           .to have_http_status(200)
           .and have_cacheable_headers
-          .and have_attributes(
-            content_type: match('text/css')
-          )
-        expect(response.body.presence)
-          .to be_nil
+        expect(response.media_type)
+          .to eq('text/css')
+        expect(response.body)
+          .to be_blank
       end
     end
 
@@ -32,9 +31,8 @@ RSpec.describe 'Custom CSS' do
         expect(response)
           .to have_http_status(200)
           .and have_cacheable_headers
-          .and have_attributes(
-            content_type: match('text/css')
-          )
+        expect(response.media_type)
+          .to eq('text/css')
         expect(response.body.strip)
           .to eq(expected_css)
       end