c# - How to send html in attachment?

I need to send some text in html by email. But I need to do it not in the email body but in the attachment. I use this code snippet to form an attachment:

public MailMessage GetMailMessage(string @from)
{
  var m = new MailMessage(new MailAddress(@from), new MailAddress(_to)) {Subject = _subj};
  var attach = new Attachment(new MemoryStream(
       Encoding.UTF8.GetBytes(_htmlAttach)), _subj + ".html", MediaTypeNames.Text.Html);
  attach.ContentDisposition.Inline = false;
  attach.ContentType.Name = _subj + ".html";
  m.Attachments.Add(attach);
  return m;

}

But when I send such an email to my gmail mailbox i recieve a plain-text message with smth like this in text:

\ =?utf-8?B?dDlDd0lOQy8wTFhSZ05DNDBMN1F0Q0F4Pz0NCiA9P3V0Zi04P0I/TXpv?=\ \ =?utf-
8?B?ME5pQXlPUzR3T0M0eU1ERXhJQzBnTVRRNk1qQWdNamt1TURndU1qQXhN?=\ \ =?utf-8?B?UzVvZEcxcz89?=" 
Content-Transfer-Encoding: base64 Content-Disposition: attachment 
PCFET0NUWVBFIGh0bWwgUFVCTElDICItLy9XM0MvL0RURCBIVE1MIDQuMDEvL0VO 
IiAiaHR0cDovL3d3dy53My5vcmcvVFIvaHRtbDQvc3RyaWN0LmR0ZCI+DQo8aHRt 

bCB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMTk5OS94aHRtbCIgeG1sOmxhbmc9 ImVuIj4NCjxoZWFkPg0KI

What is the right way to send a html-file in attach?

---

UPD:Here is the raw mail message part:

MIME-Version: 1.0

From: 
To: xxx@gmail.com
Date: Mon, 29 Aug 2011 03:29:05 -0700 (PDT)
Subject: xxx
Content-Type: multipart/mixed; boundary=--boundary_0_c99e5172-1766-4cfe-a6e0-7f6b0fa11061
----boundary_0_c99e5172-1766-4cfe-a6e0-7f6b0fa11061
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable



----boundary_0_c99e5172-1766-4cfe-a6e0-7f6b0fa11061
Content-Type: text/html;
name="=?utf-8?B?PT91dGYtOD9CPzBLUFFzdEMxMExUUXZ0QzgwTHZRdGRDOTBMalF0U0RR?=\
\

 =?utf-8?B?dDlDd0lOQy8wTFhSZ05DNDBMN1F0Q0F4Pz0NCiA9P3V0Zi04P0I/TXpv?=\
\

 =?utf-8?B?ME5pQXlPUzR3T0M0eU1ERXhJQzBnTVRRNk1qQWdNamt1TURndU1qQXhN?=\
\

 =?utf-8?B?UzVvZEcxcz89?="
Content-Transfer-Encoding: base64
Content-Disposition: attachment

Then goes the base64 text:

PCFET0NUWVBFIGh0bWwgUFVCTElDICItLy9XM0MvL0RURCBIVE1MIDQuMDEvL0VO
IiAiaHR0cDovL3d3dy53My5vcmcvVFIvaHRtbDQvc3RyaWN0LmR0ZCI+DQo8aHRt
bCB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMTk5OS94aHRtbCIgeG1sOmxhbmc9

ImVuIj4NCjxoZWFkPg0KICAgIDxtZXRhIGNvbnRlbnQ9InRleHQvaHRtbDsgY2hh

And in the end:

----boundary_0_c99e5172-1766-4cfe-a6e0-7f6b0fa11061--