FCK editor does not show PDF files

I use FCK editor with Cubecart. when I try to add a link to a PDF file, it does not show up in the display window. i can select it and upload the file. After uploaded, it does not show up, but if I try to upload again, it realizes there is a file there with the same name, just not displaying.

I have tried to adjust the allowed and denied file types, but can you tell me exactly where these permissions might be?

Or some other ideas.

thanks.