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.

lib/file_store/s3_store.rb

@@ -98,6 +98,7 @@ module FileStore
       # if this fails, it will throw an exception
       if opts[:move_existing] && opts[:existing_external_upload_key]
         original_path = opts[:existing_external_upload_key]
+        options[:apply_metadata_to_destination] = true
         path, etag = s3_helper.copy(
           original_path,
           path,