"prova" e' il nome del form... non gli puoi attribuire un value :quipy:

<form name="prova" action="carica_database.php" method="GET">
<input type=submit style='width:100%;' value='Carica database'>
<input type="file" name="database"
onchange="this.form.pippo.value=this.value.substr( this.value.lastIndexOf('\\')+1)">
<input type=text name="pippo">
</form>

ciao