you should apply a valid and EXISTING !! folder on the SERVER from where to get the attach file. Usually this is the Upload folder path.
you should get it that way :
string ValidServerPath = Server.MapPath(string.Empty) + @"\" + UPLOAD_FOLDER + @"\";
STEP 2 - Get the valid path for the attach file
supply the attach file name and concat it to that path, like that :
string validServerFilePath = ValidServerPath + sAttachedFileName;
STEP 3 - Set encoding - IMPORTANT !!
you have to set the mail encoding to handle all types of extentions (.doc, .txt, .jpg ...)
to do that, create the MailAttachment object like that :
MailAttachment fileAttachment = new MailAttachment(validServerFilePath, MailEncoding.UUEncode);
STEP 4 - Attach the file to the message
myMsg.Attachments.Add(fileAttachment);