private static String encodeRFC5987(final String filename) {
        return "UTF-8''" + PercentCodec.RFC5987.encode(filename);
    }
Encodes the given filename according to RFC 5987, prefixing it with {@code UTF-8''} and
applying percent-encoding to non-ASCII characters. This is used for the {@code filename*}
parameter in the {@code Content-Disposition} header when non-ISO-8859-1 characters are present.

@param filename the filename to encode, must not be {@code null}
@return the RFC 5987-encoded string, e.g., {@code UTF-8''example%20text}
@since 5.5
httpclient5/src/main/java/org/apache/hc/client5/http/entity/mime/FormBodyPartBuilder.java