Попробуйте сделать это программно
/YOURWEB/_layouts/WordViewer.aspx?id= "here is query string (path to the document)"
или
<SharePoint:MenuItemTemplate ID="submit1" runat="server" Text="open" ImageUrl="/_layouts/images/OpeninBrowser.gif" ClientOnClickScript="OpenWebApps('%Url%'); return false;">
function OpenWebApps(url) {
var ext = url.substr(url.lastIndexOf('.') + 1); // here define the extension
if (ext == 'docx')
window.open('<%= Web.Url%>'+"/_layouts/WordViewer.aspx?id="+encodeURI(url));
else
alert('Cannot open document with this extension');
}