Opening attachments

HelloI have a frameset of two frames. In left frame i opens a document i right i open the attachment. The right frame is set to open URL and URL is created with Formula:

st:=“http://server/database/0/”;

ud:=@Text(@DocumentUniqueID);

file:=@AttachmentNames;

st+ud+“/$FILE/”+file

All is OK when the attachment is jpg,bmp or gif file. But when attachment is a pdf file it shows a dialog to choose folder with plug-ins. I tried changing the browsers and then i choose the plug-ins folder of this browsers but it doesn’t help

Somebody can help me?