[SATLUG] predefined helper application

[Borries Demeler]

I have a question about Mozilla:

Whenever I download an attachment of type application/octet-stream that is
a PDF file, Mozilla wants to use this atrocious GNOME PDF viewer to display 
it. When I go into helper applications the entry for this MIME type doesn't
list GNOME PDF viewer at all and any attempts to override it with acroread
are useless. So my question:

WHERE can I redefine this selection?

Thanks, -borries