The best way to find out what's going on, and a good programming practice in general, is to add better error checking:

use strict;
use CGI;

my $q = CGI->new()
  or die "Couldn't create CGI object!\n";

sub mimetype {
   my $file = shift;
   my $info = $q->uploadInfo($file)
     or die "No file '$file' was uploaded!\n";
   my $mimetype = $info->{'Content-Type'} # line 804
     or die "Uploaded file had no Content-type!\n";
   return $mimetype;
}
[download]