Support varying ICO files

Closes #32096.

activestorage/app/models/active_storage/variant.rb

@@ -115,7 +115,7 @@ class ActiveStorage::Variant
 
     def download_image
       require "mini_magick"
-      MiniMagick::Image.create { |file| download_blob_to(file) }
+      MiniMagick::Image.create(blob.filename.extension_with_delimiter) { |file| download_blob_to(file) }
     end
 
     def transform(image)

activestorage/lib/active_storage/engine.rb

@@ -18,7 +18,15 @@ module ActiveStorage
     config.active_storage.analyzers = [ ActiveStorage::Analyzer::ImageAnalyzer, ActiveStorage::Analyzer::VideoAnalyzer ]
     config.active_storage.paths = ActiveSupport::OrderedOptions.new
 
-    config.active_storage.variable_content_types = %w( image/png image/gif image/jpg image/jpeg image/vnd.adobe.photoshop )
+    config.active_storage.variable_content_types = %w(
+      image/png
+      image/gif
+      image/jpg
+      image/jpeg
+      image/vnd.adobe.photoshop
+      image/vnd.microsoft.icon
+    )
+
     config.active_storage.content_types_to_serve_as_binary = %w(
       text/html
       text/javascript

activestorage/test/models/variant_test.rb

@@ -50,6 +50,17 @@ class ActiveStorage::VariantTest < ActiveSupport::TestCase
     assert_equal 20, image.height
   end
 
+  test "resized variation of ICO blob" do
+    blob = create_file_blob(filename: "favicon.ico", content_type: "image/vnd.microsoft.icon")
+    variant = blob.variant(resize: "20x20").processed
+    assert_match(/icon\.png/, variant.service_url)
+
+    image = read_image(variant)
+    assert_equal "PNG", image.type
+    assert_equal 20, image.width
+    assert_equal 20, image.height
+  end
+
   test "optimized variation of GIF blob" do
     blob = create_file_blob(filename: "image.gif", content_type: "image/gif")