text/rtf should be application/rtf

Hello,

While investigating a bug report from a user, I came up against the treatment of RTF documents in webfm/webfm_mime_types.inc
.

Per my reading of http://www.iana.org/assignments/media-types and /etc/mime.types on linux, the following MIME types should go with the following extensions;

application/rtf rtf
text/richtext rtx
text/rtf [none]

Thus I believe you should be serving .rtf's as application/rtf.