class dlFiles {
  ".exe"
  ".pdf"
  ".vb"
}

when HTTP_REQUEST {
  # Compare the file extension to the list you wish to force downloads for
  if { [matchclass [string tolower [HTTP::uri]] ends_with $::dlFiles] } {
    set forceDownload 1
  }
}
 
when HTTP_RESPONSE {  
  if { $forceDownload == 1 } {
    # Check if the response has a Content-Disposition header  
    if {[HTTP::header exists Content-Disposition]}{  
      # Replace the existing Content-Disposition header  
      # with one that prompts the user to download the content  
      HTTP::header replace Content-Disposition "attachment; filename="  
    }
    set forceDownload 0  
  }
}