A mapping between `.rtf` and `application/rtf` [is present](https://github.com/samuelneff/MimeTypeMap/blob/bf7c10659654c6131552c992bf9b1fa0b41a6f48/MimeTypeMap.cs#L473), but `.rtf` can also be associated to `text/rtf` (see: https://www.iana.org/assignments/media-types/text/rtf) Can that also be added?