string fileName = "38374-fjf(1).13jk";
string contentDisposition = string.Format("attachment;filename={0}",fileName);
msg.Content.Headers.ContentDisposition = ContentDispositionHeaderValue.Parse(contentDisposition);

5.  Examples

   Direct the UA to show "save as" dialog, with a filename of
   "example.html":

     Content-Disposition: Attachment; filename=example.html

   Direct the UA to behave as if the Content-Disposition header field
   wasn‘t present, but to remember the filename "an example.html" for a
   subsequent save operation:

     Content-Disposition: INLINE; FILENAME= "an example.html"

   Note: This uses the quoted-string form so that the space character
   can be included.

   Direct the UA to show "save as" dialog, with a filename containing
   the Unicode character U+20AC (EURO SIGN):

     Content-Disposition: attachment;
                          filename*= UTF-8‘‘%e2%82%ac%20rates

   Here, the encoding defined in [RFC5987] is also used to encode the
   non-ISO-8859-1 character.

   This example is the same as the one above, but adding the "filename"
   parameter for compatibility with user agents not implementing
   RFC 5987:

     Content-Disposition: attachment;
                          filename="EURO rates";
                          filename*=utf-8‘‘%e2%82%ac%20rates

   Note: Those user agents that do not support the RFC 5987 encoding
   ignore "filename*" when it occurs after "filename".

string contentDisposition = string.Format("attachment;filename=\"{0}\"", fileName);

string fileName= HttpUtility.UrlEncode("啊fnam(4)e.ext",UTF8Encoding.UTF8);