[plug] Prevent downloads

[Craig Ringer]

> The PC's in the Telecentre in question need to access Hotmail for tourists
> and the Internet on a Read and (maybe) Print only basis .... there is no
> need to download any executable file ....but school kids are doing it !

There's nothing magic about executables, and unfortunately they're 
usually sent with MIME type application/octet-stream , same as any other 
undifferentiated data. This makes it hard to filter on them without 
blocking other things.

> So the question is "How to browse the Internet, including Hotmail, without
> allowing users to download files" - even Outlook Express type mail is not
> needed, just web mail like Hotmail.

Block all MIME types but text/plain and image/* at the proxy. If it 
can't, get a better proxy. Perhaps consider allowing specific types like 
that of PDF files, maybe Macromedia Flash .

Craig