public class AttachmentHelper
{
    public static Attachment CreateAttachment(string attachmentFile, string displayName, TransferEncoding transferEncoding)
    {
        Attachment attachment = new Attachment(attachmentFile);
        attachment.TransferEncoding = transferEncoding;
 
        string tranferEncodingMarker = String.Empty;
        string encodingMarker = String.Empty;
        int maxChunkLength = 0;
 
        switch (transferEncoding)
        {
            case TransferEncoding.Base64:
                tranferEncodingMarker = "B";
                encodingMarker = "UTF-8";
                maxChunkLength = 30;
                break;
            case TransferEncoding.QuotedPrintable:
                tranferEncodingMarker = "Q";
                encodingMarker = "ISO-8859-1";
                maxChunkLength = 76;
                break;
            default:
                throw (new ArgumentException(String.Format("The specified TransferEncoding is not supported: {0}", transferEncoding, "transferEncoding")));
        }
 
        attachment.NameEncoding = Encoding.GetEncoding(encodingMarker);
 
        string encodingtoken = String.Format("=?{0}?{1}?", encodingMarker, tranferEncodingMarker);
        string softbreak = "?=";
        string encodedAttachmentName = encodingtoken;
 
        if (attachment.TransferEncoding == TransferEncoding.QuotedPrintable)
            encodedAttachmentName = HttpUtility.UrlEncode(displayName, Encoding.Default).Replace("+", " ").Replace("%", "=");
        else
            encodedAttachmentName = Convert.ToBase64String(Encoding.UTF8.GetBytes(displayName));
 
        encodedAttachmentName = SplitEncodedAttachmentName(encodingtoken, softbreak, maxChunkLength, encodedAttachmentName);
        attachment.Name = encodedAttachmentName;
 
        return attachment;
    }
 
    private static string SplitEncodedAttachmentName(string encodingtoken, string softbreak, int maxChunkLength, string encoded)
    {
        int splitLength = maxChunkLength - encodingtoken.Length - (softbreak.Length * 2);
        var parts = SplitByLength(encoded, splitLength);
 
        string encodedAttachmentName = encodingtoken;
 
        foreach (var part in parts)
            encodedAttachmentName += part + softbreak + encodingtoken;
 
        encodedAttachmentName = encodedAttachmentName.Remove(encodedAttachmentName.Length - encodingtoken.Length, encodingtoken.Length);
        return encodedAttachmentName;
    }
 
    private static IEnumerable<string> SplitByLength(string stringToSplit, int length)
    {
        while (stringToSplit.Length > length)
        {
            yield return stringToSplit.Substring(0, length);
            stringToSplit = stringToSplit.Substring(length);
        }
 
        if (stringToSplit.Length > 0) yield return stringToSplit;
    }
}