An Attachment object is NOT the actual attachment file (the source file). You can create an attachment object to represent the attachment file, then add the object to a feature for later upload. On the other hand, for the attachments already existing on server side, you can choose to download some or all of the actual attachment files. However, information about all attachments is always downloaded together with features in a feature download session.

To utilize the attachment functionality, you need to first enable attachments on the feature source in the map document. Global Ids also need to be added to the FeatureSource and the attachment table. In addition, the mobile service needs to be hosted in a 10.1 ArcGIS Server.

System.Object
   ESRI.ArcGIS.Mobile.FeatureCaching.Attachment

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