public long AddAttachment( 
   Attachment attachment
)

public async Task AddingAttachments()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.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);
            }
          }
        }
      }
    }
  });
}

private 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;
}