isset's don't work on images?

This doesn't seem to work... isset only works for variables? My image uploads fine if I remove this code, how do you test if a file has been uploaded or not?

if (isset($_POST[imagefile])){ }



<input type=\"file\" name=\"imagefile\">

how do you test if a file has been uploaded or not
http://www.php.net/manual/en/features.file-upload.errors.php

Sweet, thanks!