How to get attachment with owaIDs?

Hello,

I'm working on a project which allows user to save attachment to user selected location on remote server. For example, a student uses OWA to check his emails at home, he can save attachments in the folders located on the remote server at school.

What I can get on OWA side are:
owaID of the message like:
_sId="RgAAAAB7RLJw1vq4TZJ+Ug2A4QY1BwA+Mfi+BzbkSbxcWw/Wo0gQAAAAAAAUAAA+Mfi+BzbkSbxcWw/Wo0gQAAGCKytpAAAJ"
and owaID of attachment as:
_attid="BAAAAAAA"

I can convert message owaID to ewsID and get the message object. Then, I can loop through the attachments of the message. However, I don't know how to get the ewsId for the selected attachment with _attid="BAAAAAAA". When I try to convert "BAAAAAAA" to ewsId, it returns "data corrupted" error.

Here is my function to convert owaId to ewsId:
public string ConvertOwaIdtoEWSid(ExchangeService ews, string mailboxSMTP, string idToConvert)
{
    string retVal = null;
    try
    {
 AlternateId originalId = new AlternateId(IdFormat.OwaId, idToConvert, mailboxSMTP);
 AlternateId newId = ews.ConvertId(originalId, IdFormat.EwsId) as AlternateId;
 retVal = newId.UniqueId;
    }
    catch (Exception ex)
    {
 retVal = ex.Message;
    }
    return retVal;
}

Any response would be appreciated. Thanks.