public long AddAttachment( 
   Attachment attachment
)

{
  await QueuedTask.Run(() =>
  {
    using (Geodatabase geodatabase = new Geodatabase(new DatabaseConnectionFile(new Uri("path\\to\\sde\\file\\sdefile.sde"))))
    using (FeatureClass parkFeatureClass = geodatabase.OpenDataset<FeatureClass>("LocalGovernment.GDB.Park"))
    {
      QueryFilter filter = new QueryFilter { WhereClause = "NUMPARKING > 0" };

      using (RowCursor parkingCursor = parkFeatureClass.Search(filter, false))
      {
        while (parkingCursor.MoveNext())
        {
          using (MemoryStream stream = CreateMemoryStreamFromContentsOf("Sample.xml"))
          {
            Attachment attachment = new Attachment("Sample.xml", "text/xml", stream);

            using (Row row = parkingCursor.Current)
            {
              long attachmentId = row.AddAttachment(attachment);
            }
          }
        }
      }
    }
  });
  // Helper method to read a file into a memory stream
  MemoryStream CreateMemoryStreamFromContentsOf(String fileNameWithPath)
  {
    MemoryStream memoryStream = new MemoryStream();

    using (FileStream file = new FileStream(fileNameWithPath, FileMode.Open, FileAccess.Read))
    {
      byte[] bytes = new byte[file.Length];
      file.Read(bytes, 0, (int)file.Length);
      memoryStream.Write(bytes, 0, (int)file.Length);
    }

    return memoryStream;
  }
}