Why can't you use the "Goto page view"? This is the preferred method because it will work when JS is not available. Just open the attachment in another Acrobat tab and when you select the "Goto page view" action, click on the embedded file.
The JavaScript for this is tricky, but here it is:
var oDoc = openDataObject(dataObjects[0].name)
app.openDoc(xx.path)
This code opens the first file attachment. And it won't work everywhere.