HTML not posting file to PHP

Here is my code:

<html>
<body>

<div>
    <form action="<?=$_SERVER['PHP_SELF'];?>" method="post">
      <input type="file" id="myFile" name="filename">
      <input type="submit" name="Submit">
    </form>
</div>


<?php
if(isset($_POST['Submit']))
{
    $file = $_POST["myFile"];
    echo("hello");
}


?>

</body>
</html>

when I run it and input a file it says: Undefined array key “myFile”

what am I doing wrong?