to figure out the correct MIME type for the file, you could use the File::MMagic module (there's no way i would use the MIME type sent in the client HTTP request, since many browsers these days seem to be horribly misconfigured w.r.t. MIME). i'm not sure how well it works under non-UNIX platforms, but it does have an 'external file' mode which i believe could be used under Windows with the /etc/magic file from a Linux distro, if necessary.

if you can't figure out what the correct MIME type is, it is probably better to use application/octet-stream instead of unknown/unknown.