Attachment name in Outlook looks wrong (encoding issue?) - outlook

Question

I hope someone can point us out to the issue in our code. Take a look at the picture.
As you can see the attachment name is ALMOST as expected. We want the name to be 'Anmälan-A00045.pdf' and 'Anmälan-45tg.pdf'. If I save the attachment the name is shown correctly. I believe that I am setting the encoding wrong somewhere. Trying to dump the mail as a file leads to the following output.
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="8EC249FE-78A0-420A-98D9-D953E58A8F9E"
--8EC249FE-78A0-420A-98D9-D953E58A8F9E
MIME-Version: 1.0
Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: 7bit
Content-ID: {4EE462B2-16FB-47AE-A318-1C2C6ED889D3}
Content-Description: body
(body of email above and below is the attachment stuff)
--8EC249FE-78A0-420A-98D9-D953E58A8F9E
MIME-Version: 1.0
Content-Type: application/pdf
Content-Transfer-Encoding: base64
Content-ID: {F4F32EDB-36AF-4DAE-982A-93C3BF0F59C6}
Content-Description: Anmälan-45tg.pdf
Content-Disposition: attachment;
filename="=?utf-8?B?QW5tw6RsYW4tNDV0Zy5wZGZ=?="
We are using Biztalk with a custom send pipeline (containing our pipeline component and the mime/smime in encoding stage) to send out an email using SMTP-adapter. Now, as with many other posts in SO, this works perfect in other email clients like Gmail or Hotmail.
Now, code snippet below is what I am using to set the attachments and everything related to it. Does anyone have an idea of what to try? My trials with the method HtmlEncode did nothing, unfortunately.
public void ProcessAttachments(IPipelineContext _pContext, IBaseMessage _msg,
IList<Attachment> _attachments)
{
foreach (var item in _attachments)
{
var msgPart = _pContext.GetMessageFactory().CreateMessagePart();
msgPart.PartProperties.Write("FileName", "http://schemas.microsoft.com/BizTalk/2003/mime-properties", item.FileName + item.Extension);
msgPart.PartProperties.Write("ContentDescription", "http://schemas.microsoft.com/BizTalk/2003/mime-properties", item.ContentType);
// MIME / SMIME content transfer encoding
msgPart.PartProperties.Write("ContentTransferEncoding", "http://schemas.microsoft.com/BizTalk/2003/mime-properties", Encoding.UTF8);
msgPart.Data = new MemoryStream(BytesFromBase64String(item.Base64));
msgPart.ContentType = item.ContentType;
//msgPart.Charset = "UTF-8";
//_msg.AddPart(HtmlEncode(item.FileName + item.Extension), msgPart, false);
_msg.AddPart(item.FileName + item.Extension, msgPart, false);
}
}
private static string HtmlEncode(string filename)
{
/* Old code. What this does is change 'ä' to 'ä' Matters little as Outlook intreprets it literally. The filename in Outlook is set as 'Anmälan-45tg.pdf'.
string result;
using (StringWriter sw = new StringWriter())
{
var x = new HtmlTextWriter(sw);
x.WriteEncodedText(filename);
result = sw.ToString();
}
return result;
*/
var utf8 = Encoding.UTF8;
byte[] utfBytes = utf8.GetBytes(filename);
return utf8.GetString(utfBytes);
}