Users downloading non-ASCII attachments would see garbled characters.
When used with object storage, AWS S3 would return an InvalidArgument
error: Header value cannot be represented using ISO-8859-1.
Per RFC 5987 and RFC 6266, Content-Disposition should be encoded
properly. This commit takes the Rails 6 implementation of
(https://github.com/rails/rails/pull/33829) and ports it here.