Some HTTP headers support extended parameters, parameters with a "" after them which allow character encoding of the header value, e.g. in UTF-8. Confusingly, t

Some HTTP headers support extended parameters, parameters with a "" after them which allow character encoding of the header value, e.g. in UTF-8. Confusingly, they also support sending both regular and extended parameters in the same header. https://tools.ietf.org/html/rfc5987 E.g. sending the file "naïve.txt" using the Content-Disposition header. Content-Disposition: attachment; filename=na_ve.txt; filename*=utf8''na%C3%AFve.txt https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Co... The parameters filename and filename* differ only in that filename* uses the encoding defined in RFC 5987. When both filename and filename* are present in a single header field value, filename* is preferred over filename when both are understood.