Add attachments using REST with non ASCII characters

I want to add attachments to an issue using the REST Interface. If the filenames of the attachments have only ASCII characters erverything works well, but if the filename containes non ASCII characters I got corrupt file names in the JIRA ticket.

For ASCII only file names my application sends:

Content-Disposition: form-data; name=file; filename=test.jpg; filename*=utf-8''test.jpg

For file names with non ASCII characters it sends:

Content-Disposition: form-data; name=file; filename"=?utf-8?B?YcOkb8O2dcO8w58gw5zDlsOEdGVzdEDigqwuanBn?="; filename*=utf-8''a%C3%A4o%C3%B6u%C3%BC%C3%9F%20%C3%9C%C3%96%C3%84test%40%E2%82%AC.jpg

As specified in RFC 1867 and RFC 822.

I also tried "=?utf-8?Q? and it does also not work.

So is it a bug in JIRA or is it the wrong way to handle filenames with non ASCII characters?