Few things that you can try out:
1> Try compacting the database (long shot) and also run a fixup.
2> On your web form try using @Attachmentnames somewhere in a <computed value>just to test if it is picking up the right attachment or not (although i know if you right click it opens fine in a new window)
3> Try refreshing the document using some formula like @Command([ToolsRefreshSelectedDocs])
4> Design a $$Viewtemplate form for that view and then get the attachment displayed by using the correct url syntax like http://www.yourserver.com/abc/test.nsf/0/