FIX: Make sure S3 object headers are preserved on copy (#14302)

When copying an existing upload stub temporary object
on S3 to its final destination we were not copying across
its additional headers such as content-disposition and
cache-control, which led to issues like attachments not
downloading with their original filename when clicking
the download links in posts.

This is because the metadata_directive = REPLACE option
was not being passed to object.copy_from(), so only the
source object's headers were being used. Added an option
for apply_metadata_to_destination to apply this option
conditionally, because we may not always want to replace
this metadata, but we definitely do when copying a temporary
upload.

spec/components/s3_helper_spec.rb

@@ -95,4 +95,38 @@ describe "S3Helper" do
     expect(object1.key).to eq("folder_path/original/1X/def.xyz")
     expect(object2.key).to eq("#{FileStore::BaseStore::TEMPORARY_UPLOAD_PREFIX}folder_path/uploads/default/blah/def.xyz")
   end
+
+  describe "#copy" do
+    let(:source_key) { "#{FileStore::BaseStore::TEMPORARY_UPLOAD_PREFIX}uploads/default/blah/source.jpg" }
+    let(:destination_key) { "original/1X/destination.jpg" }
+    let(:s3_helper) { S3Helper.new("test-bucket", "", client: client) }
+
+    it "can copy an object from the source to the destination" do
+      destination_stub = Aws::S3::Object.new(bucket_name: "test-bucket", key: destination_key, client: client)
+      s3_helper.send(:s3_bucket).expects(:object).with(destination_key).returns(destination_stub)
+      destination_stub.expects(:copy_from).with(copy_source: "test-bucket/#{source_key}").returns(
+        stub(copy_object_result: stub(etag: "\"etag\""))
+      )
+      response = s3_helper.copy(source_key, destination_key)
+      expect(response.first).to eq(destination_key)
+      expect(response.second).to eq("etag")
+    end
+
+    it "puts the metadata from options onto the destination if apply_metadata_to_destination" do
+      content_disposition = "attachment; filename=\"source.jpg\"; filename*=UTF-8''source.jpg"
+      destination_stub = Aws::S3::Object.new(bucket_name: "test-bucket", key: destination_key, client: client)
+      s3_helper.send(:s3_bucket).expects(:object).with(destination_key).returns(destination_stub)
+      destination_stub.expects(:copy_from).with(
+        copy_source: "test-bucket/#{source_key}", content_disposition: content_disposition, metadata_directive: "REPLACE"
+      ).returns(
+        stub(copy_object_result: stub(etag: "\"etag\""))
+      )
+      response = s3_helper.copy(
+        source_key, destination_key,
+        options: { apply_metadata_to_destination: true, content_disposition: content_disposition }
+      )
+      expect(response.first).to eq(destination_key)
+      expect(response.second).to eq("etag")
+    end
+  end
 end