FIX: Abort theme creation if unable to create uploads (#16336)

Previous to this change if any of the assets were not allowed extensions
they would simply be silently ignored, this could lead to broken themes
that are very hard to debug

app/models/remote_theme.rb

@@ -162,6 +162,11 @@ class RemoteTheme < ActiveRecord::Base
         new_path = "#{File.dirname(path)}/#{SecureRandom.hex}#{File.extname(path)}"
         File.rename(path, new_path) # OptimizedImage has strict file name restrictions, so rename temporarily
         upload = UploadCreator.new(File.open(new_path), File.basename(relative_path), for_theme: true).create_for(theme.user_id)
+
+        if !upload.errors.empty?
+          raise ImportError, I18n.t("themes.import_error.upload", name: name, errors: upload.errors.full_messages.join(","))
+        end
+
         updated_fields << theme.set_field(target: :common, name: name, type: :theme_upload_var, upload_id: upload.id)
       end
     end

config/locales/server.en.yml

@@ -72,6 +72,7 @@ en:
       unrecognized_extension: "Unrecognized file extension: %{extension}"
     import_error:
       generic: An error occurred while importing that theme
+      upload: "Error creating upload asset: %{name}. %{errors}"
       about_json: "Import Error: about.json does not exist, or is invalid. Are you sure this is a Discourse Theme?"
       about_json_values: "about.json contains invalid values: %{errors}"
       modifier_values: "about.json modifiers contain invalid values: %{errors}"

spec/requests/admin/themes_controller_spec.rb

@@ -151,6 +151,16 @@ describe Admin::ThemesController do
       expect(UserHistory.where(action: UserHistory.actions[:change_theme]).count).to eq(1)
     end
 
+    it 'fails to import with an error if uploads are not allowed' do
+      SiteSetting.theme_authorized_extensions = "nothing"
+
+      expect do
+        post "/admin/themes/import.json", params: { theme: theme_archive }
+      end.to change { Theme.count }.by (0)
+
+      expect(response.status).to eq(422)
+    end
+
     it 'imports a theme from an archive' do
       _existing_theme = Fabricate(:theme, name: "Header Icons")