ho provato anche io in una pagine nuova e funziona ma non capisco perchè in questapagina no :master:
codice:
<SCRIPT language="Javascript1.2">
function check()
{
var x=document.form1.elements[0].value;
var ext=x.length;
document.form1.elements[1].value=250-ext;
if(parseInt(document.form1.elements[1].value)<0) 
{
document.form1.elements[1].value="Err";
window.alert("Puoi scrivere al massimo 250 caratteri!!!");
var shorter=x.slice(0,250); 
document.form1.elements[0].value=shorter;
document.form1.elements[1].value="0";
}
}

</SCRIPT>
questo è il form1

codice:
<form action="<?php echo $editFormAction; ?>" method="post" enctype="multipart/form-data" name="form1">
    <table align="center">
      <tr valign="middle">
        <td align="right" nowrap class="Stile7"><span class="Stile7">Nome:</span></td>
        <td><div align="left">
          <input name="nome" type="text" value="" size="50" tabindex="1">
        </div></td>
      </tr>
      <tr valign="middle">
        <td align="right" nowrap class="Stile7"><span class="Stile7">Descrizione:</span></td>
        <td><div align="left">
          <textarea name="descrizione" id="descrizione" onkeyup="check()"></textarea>
        </div></td>
      </tr>
      <tr valign="middle">
        <td align="right" nowrap class="Stile7"><span class="Stile7">Peso:</span></td>
        <td><div align="left">
          <input name="peso" type="text" value="" size="10">
          <span class="Stile7">espresso in KB </span></div></td>
      </tr>
      <tr valign="middle">
        <td align="right" nowrap class="Stile7">Collegamento</td>
        <td><div align="left">
          <input name="url" type="text" id="url" value="http://www.miosito.it" size="50">
        </div></td>
      </tr>
      <tr valign="middle">
        <td align="right" nowrap class="Stile7"><INPUT TYPE="hidden" name="MAX_FILE_SIZE" value="1024000">
          Miniatura          </td>
        <td>
          <div align="left">
            <input name="UploadedFile" type="file" onChange="document.form1('miniatura').value=this.value.substr(this.value.lastIndexOf('\\')+1)" size="50">
          </div></td>
      </tr>
      <tr valign="middle">
        <td align="right" nowrap class="Stile7"><INPUT TYPE="hidden" name="MAX_FILE_SIZE" value="1024000">
        Ingrandimento</td>
        <td><div align="left">
          <input name="UploadedFileing" type="file" onChange="document.form1('ingrandimento').value=this.value.substr(this.value.lastIndexOf('\\')+1)" size="50">
        </div>
      </tr>
      <tr valign="middle">
        <td colspan="2" align="right" nowrap class="Stile7"></td>
      </tr>
      <tr valign="middle">
        <td colspan="2" align="right" nowrap class="Stile7">
          <div align="center"><span class="Stile7">
            <input name="miniatura" type="hidden" id="miniatura">
            <input name="ingrandimento" type="hidden" id="ingrandimento">
            </span>
            <input name="datains" type="hidden" id="datains" value="<?php echo date('Y-m-j') ?>">
            <input type="submit" value="Inserisci">
          </div></td>
      </tr>
    </table>
    <input type="hidden" name="MM_insert" value="form1">
  </form>