Before attachment information can be manipulated, you need to add an Attachment object to the FeatureSource via an AttachmentManager. This will copy or move the actual attachment file to the cache folder and link it to the attachment information.

Note that attachments can only be added to features already existing in database or in delta table. Therefore, newly added features must be saved locally before attachments can be added to them.

System.Object
   ESRI.ArcGIS.Mobile.FeatureCaching.AttachmentManager

Target Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, Windows Vista, Windows Server 2008 family

Reference