When specifying an attachment's name, you need to put a file extension for it (e.g. name.jpg), which should match with the actual content type of the attachment file.

Note that attachment name is not the storage path of the attachment, and attachments added to a feature can have the same name.

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