<form name="elformu" method="POST" enctype="multipart/form-data" onsubmit="ve()" action="http://xxx.php?">
<p><input type="file" name="F1" size="20">
<input type="submit" value="Enviar" name="B1"></p>
</form>
<script>
function ve(){
var elfichero= elformu.F1.value;
alert(elfichero)}
</script>