...

       When a file is uploaded the browser usually sends along
       some information along with it in the format of headers.
       The information usually includes the MIME content type.
       Future browsers may send other information as well (such
       as modification date and size). To retrieve this
       information, call uploadInfo().  It returns a reference to
       an associative array containing all the document headers.

              $filename = $query->param('uploaded_file');
              $type = $query->uploadInfo($filename)->{'Content-Type'};
              unless ($type eq 'text/html') {
                 die "HTML FILES ONLY!";
              }