$_FILES IS EMPTY AFTER POST FORM

when i try to POST a file using a simple form like:

<form action="up.php" enctype="multipart/form-data" method="post">
   <?
    echo "Nome <input type='text' name='exname' value='DD' size='15' />";   
    echo '</br>';
    echo "Séries <input type='text' name='serie' value='DDSD' size='15' />";
    echo '</br>';
    echo "Repetições <input type='text' name='reps' value='SDS' size='15' />";
    echo '</br>';

    echo "Peso <input type='text' name='peso' value='4' size='15' />";
    echo '</br>';
    echo "Foto   <input type='file' name='foto' id='fileToUpload' onchange='fileSelected();' accept='image/*' capture='camera' />";

?>
    <button type='submit'>Salvar</button>

it works fine and i can get the $FILES content. i have already read a lot about those problems with POST and FILES, but i could not get a page working...($_FILES is ALWAYS empty after post) check my code please:

<form action="login.php?setexe" enctype="multipart/form-data" method="post">

        <div data-role="content" id="about-content">
            <div class="ui-body ui-body-c">


                    <h3>Alterar Exercício</h3>

                         <fieldset data-role="controlgroup">
                <?php
                $number = preg_replace("/[^0-9]/","",$exe); 
                echo "<input type='hidden' name='ida' value='$id'/>";
                echo "<input type='hidden' name='n' value='$exe'/>";

                $con = new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME);

                $t = $con->query("SELECT * FROM $table WHERE id = '$id'");
                while($line = $t->fetch_array()) {

                echo "Nome <input type='text' name='exname' value='".$line['ex'.$number.'']."' size='15' />";   
                echo '</br>';
                echo "Séries <input type='text' name='serie' value='".$line['ser'.$number.'']."' size='15' />";
                echo '</br>';
                echo "Repetições <input type='text' name='reps' value='".$line['rep'.$number.'']."' size='15' />";
                echo '</br>';

                echo "Peso <input type='text' name='peso' value='".$line['peso'.$number.'']."' size='15' />";
                echo '</br>';

                echo "Observação <input type='text' name='obs' value='".$line['obs'.$number.'']."' size='15' />";
               ?>

               Foto <input type="file" name="fotoex" id="fileToUpload" onchange="fileSelected();" accept="image/*" capture="camera" />
    <?
                }   
                echo "</fieldset>";

                ?>          

                <div class='ui-block-b'><button type='submit' data-theme='a'>Salvar</button></div>





                    </td>



                </div>




                </div>
            </form> 

Thank you!