iRule cannot fetch iFile (wpad.dat)

Your iRule is changing the requested URI in lowercase then your first match case contains upper case characters, try the following:

when HTTP_REQUEST {
  switch -glob [string tolower [HTTP:uri]] {
    "/mdd/wpad.dat" {
      HTTP::respond 200 contain [ifile get /MDD/wpad.dat] "Content-Type" "application/x-ns-proxy-autoconfig" "pragma" "no-cache"
    }
    default {
      HTTP::respond 404 content "You've asked for something we don't have."
    }  
  }
}