Serve SVGs with correct content type (#763)

Closes
RaspberryPiFoundation/digital-editor-issues#1286

Author: cocomarine

app/controllers/api/scratch/assets_controller.rb

@@ -3,12 +3,15 @@
 module Api
   module Scratch
     class AssetsController < ScratchController
+      include ActiveStorage::SetCurrent
+
       skip_before_action :authorize_user, only: [:show]
       skip_before_action :check_scratch_feature, only: [:show]
 
       def show
         filename_with_extension = "#{params[:id]}.#{params[:format]}"
-        redirect_to rails_storage_redirect_url(ScratchAsset.find_by!(filename: filename_with_extension).file)
+        scratch_asset = ScratchAsset.find_by!(filename: filename_with_extension)
+        redirect_to scratch_asset.file.url(content_type: scratch_asset.file.content_type), allow_other_host: true
       end
 
       def create

spec/features/scratch/creating_and_showing_a_scratch_asset_spec.rb

@@ -4,22 +4,43 @@
 
 RSpec.describe 'Creating a Scratch asset', type: :request do
   let(:basename) { 'test_image_1' }
+  let(:svg_filename) { 'test_svg_image.svg' }
   let(:format) { 'png' }
   let(:filename) { "#{basename}.#{format}" }
   let(:school) { create(:school) }
   let(:teacher) { create(:teacher, school:) }
   let(:cookie_headers) { { 'Cookie' => "scratch_auth=#{UserProfileMock::TOKEN}" } }
 
   describe 'GET #show' do
-    let(:make_request) { get '/api/scratch/assets/internalapi/asset/test_image_1.png/get/' }
+    context 'when the asset is PNG' do
+      before do
+        create(:scratch_asset, :with_file, filename:, asset_path: file_fixture(filename))
+      end
 
-    context 'when the asset exists' do
-      let!(:scratch_asset) { create(:scratch_asset, :with_file, filename:, asset_path: file_fixture(filename)) }
+      let(:make_request) { get '/api/scratch/assets/internalapi/asset/test_image_1.png/get/' }
 
-      it 'redirects to the asset file URL' do
+      it 'serves the file with png content type' do
         make_request
 
-        expect(response).to redirect_to(rails_storage_redirect_url(scratch_asset.file, only_path: true))
+        follow_redirect! while response.redirect?
+
+        expect(response.media_type).to eq('image/png')
+      end
+    end
+
+    context 'when the asset is SVG' do
+      before do
+        create(:scratch_asset, :with_file, filename: svg_filename, asset_path: file_fixture(svg_filename))
+      end
+
+      let(:make_request) { get '/api/scratch/assets/internalapi/asset/test_svg_image.svg/get/' }
+
+      it 'serves the file with image/svg+xml content type' do
+        make_request
+
+        follow_redirect! while response.redirect?
+
+        expect(response.media_type).to eq('image/svg+xml')
       end
     end
   end