jquery form send only element into data: attribute

jquery form send only element into data: attribute

Sorry I have a problem, i have this form

 <form id="main_form" name="main_form" enctype="multipart/form-data" method="post">
      <div class="content">
        <ul>
            <li>
                <h3>Cognome</h3>
                <input type="text" id="main_cognome" size="20" maxlength="30" />
            </li>
            <li>
                <h3>Nome</h3>
                <input type="text" id="main_nome" size="20" maxlength="30" />
            </li>
            <li>
                <h3>Codice fiscale</h3>
                <input type="text" id="main_cf" size="20" maxlength="19" />
            </li>
            <li>
                <h3>Comeptenza</h3>
                <?php
                    echo "<select id=\"main_competenza\" >";
                    while ($row_competenza = mysql_fetch_assoc($result_competenza)) {
                        echo "<option value=\"".$row_competenza['id']."\">";
                        echo $row_competenza['desc']."</option>";
                    }
                    echo "</select>";           
                ?>
            </li>
            <li>
                <h3>Tipologia</h3>
                <?php
                    echo "<select id=\"main_tipologia\" >";
                    while ($row_tipologia = mysql_fetch_assoc($result_tipologia)) {
                        echo "<option value=\"".$row_tipologia['id']."\">";
                        echo $row_tipologia['desc']."</option>";
                    }
                    echo "</select>";           
                ?>
            </li>      
            <li>
                <h3>Allegati</h3>
                <input type="file" id="main_file" />
            </li>      
            <li>
                <h3>Inserisci Medico</h3>
                <input id="inviaMedico" type="submit" value="invia" title="Se il cognome e il nome del medico sono gi&aacute; presenti in lista verr&aacute; aumentato il numero di inadempienze anzich&eacute; inserire una riga nuova"  />
            </li>
        </ul>
      </div>
      </form>

whene try to send this form with

$(document).ready(function () {
    $('#main_form').submit(function() {
       $(this).ajaxSubmit({
        target:        "#medici",           // target element(s) to be updated with server response
        beforeSubmit:  function(formData, jqForm, options) {
                            var form = jqForm[0];
                            if (!form.main_cognome.value || form.main_cognome.value=="") {
                                $('#stato').html('errore : devi almeno inserire il cognome del medico').slideDown("slow").delay(2000).slideUp("slow");
                                $("#main_cognome").css('border', '1px dotted #c30');
                                return false;
                            }
        },              // pre-submit callback
        success:       function(responseText, statusText, xhr, $form) {
                            if($('#medicicc')) $('#medicicc').load('php/medicicc.php');
                            $("#main_cognome").css('border', 'solid 1px #009752');
                            $("input:file").MultiFile("reset");   
                            alert( $form.attr('id') );
                       },              // post-submit callback
        url:              "php/medici.php",    // override for form's 'action' attribute
        clearForm:        true
        });
        return false;
    });
});

the php page with

foreach($_POST as $var => $value)
{
echo $var . ' : ' . $value . "<br>";
}

foreach($_FILES as $file) {
    $n = $file['name'];
    $s = $file['size'];
    echo "File: $n ($s bytes)";
}

obtain nothing

if try to add attribute data : { field:value} in $_POST obtain a element!

Why I can't get element form if don't use data attribute?

please help me