Improper handling of RFC5987 HTTP parameters such as filename

I have files on my website whose names are in UTF-8 and contain characters outside the ASCII set. I am setting the Content-Disposition header as an attachment with a filename* parameter encoded per RFC5987. The non-ascii characters are translated fine, but there appears to be a problem handling space encoding (%20). Per this RFC spaces in the filename are encoded as %20, but when I download the file these encoded space characters are being converted to + characters which is incorrect. This appears to be a bug in Firefox (Chrome as well I might add).