MultipartParser incorrectly parse header values containing UTF-8 characters

I'm uploading files (using JSF) which have cyrillic names (UTF-8 characters). So in request headers i can see that as filename="some filename containing UTF-8". MultipartParser read this byte-by-byte, cast each byte to char and append to StringBuilder. It's incorrect because UTF-8 use 2 bytes to define a character. So on my side i'm getting wrong and unrecognizable filenames.