Upload script som fejler

Har følgende kode:

if($go) {
$dir = '/home/web/gb5064/filer/upload/';
$max = 1000;
$allowed = array("txt","zip","jpg","doc","JPG");

// Check to verify that the file to be uploaded was specified!
if(!isset($_FILES['file_field']['name']) || $_FILES['file_field']['name'] == "none") {
echo "<span class=normaltext>Ingen fil som kan uploades!</span><br>";
echo "<span class=normaltext>Prøv igen...</span>";
die;
}

// Check to verify if the file extension is in the allowable list or not!
$ext_array = explode( "." , $_FILES['file_field']['name'] );
$ex = $ext_array[count($ext_array) - 1];
$ex = strtolower($ex);

if(!in_array($ex,$allowed)) {
echo "<span class=normaltext>Den fil type du prøver at uploade er ikke tilladt</span>";
die;
}

// Converting the size of the file being uploaded to KB from bytes.
$_FILES['file_field']['size'] = $_FILES['file_field']['size']/1000;

// Check to verify that the filesize is less than the allowable filesize!
if($size > $max) {
echo "<span class=normaltext>Den fil du prøver at uploade er større end den tilladte: $max KB.</span>";
}

// Moving the uploaded file to the specified directory!
if(move_uploaded_file($_FILES['file_field']['tmp_name'], $dir."/".$_FILES['file_field']['name'])) {
echo "<span class=normaltext>Filen er blevet uploadet";
}
else {
echo "<span class=normaltext>Fejl... kunne ikke uploade</span>";
}
}

Den fejler hele tiden... den siger at den ikke har nogen fil at uploade så det er når den skal tjekke om der er en fil den ikke fungere... min form ser sådan her ud:

    <form action='$PHP_SELF?go=true' method='post'>
    <span class=text1>Filen som du vil uploade: </span><input type='file' name='file_field' size=30 class=form><br>
    <input type=submit value='Upload!' class=form>
    </form>