Fix Rack::Test::UploadedFile.new to work with StringIO

Instantiating Rack::Test::UplaodedFile with StringIO
no longer causes an exception.

Author: g-pavlik

activestorage/lib/active_storage/attached/changes/create_one.rb

@@ -22,8 +22,12 @@ def blob
 
     def upload
       case attachable
-      when ActionDispatch::Http::UploadedFile, Rack::Test::UploadedFile
+      when ActionDispatch::Http::UploadedFile
         blob.upload_without_unfurling(attachable.open)
+      when Rack::Test::UploadedFile
+        blob.upload_without_unfurling(
+          attachable.respond_to?(:open) ? attachable.open : attachable
+        )
       when Hash
         blob.upload_without_unfurling(attachable.fetch(:io))
       end
@@ -53,14 +57,22 @@ def find_or_build_blob
         case attachable
         when ActiveStorage::Blob
           attachable
-        when ActionDispatch::Http::UploadedFile, Rack::Test::UploadedFile
+        when ActionDispatch::Http::UploadedFile
           ActiveStorage::Blob.build_after_unfurling(
             io: attachable.open,
             filename: attachable.original_filename,
             content_type: attachable.content_type,
             record: record,
             service_name: attachment_service_name
           )
+        when Rack::Test::UploadedFile
+          ActiveStorage::Blob.build_after_unfurling(
+            io: attachable.respond_to?(:open) ? attachable.open : attachable,
+            filename: attachable.original_filename,
+            content_type: attachable.content_type,
+            record: record,
+            service_name: attachment_service_name
+          )
         when Hash
           ActiveStorage::Blob.build_after_unfurling(
             **attachable.reverse_merge(

activestorage/test/models/attached/one_test.rb

@@ -56,6 +56,15 @@ class ActiveStorage::OneAttachedTest < ActiveSupport::TestCase
     assert_equal "racecar.jpg", @user.avatar.filename.to_s
   end
 
+  test "attaching StringIO attachable to an existing record" do
+    upload = Rack::Test::UploadedFile.new StringIO.new(""), original_filename: "test.txt"
+
+    @user.avatar.attach upload
+
+    assert_not_nil @user.avatar_attachment
+    assert_not_nil @user.avatar_blob
+  end
+
   test "attaching a new blob from an uploaded file to an existing record passes record" do
     upload = fixture_file_upload("racecar.jpg")
     def upload.open