Description:
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When using an upload form, with a hidden field with the session.upload_progress.name, the target page cannot set new session variables.

If you use the example below (with session.upload_progress.cleanup set to off for debugging), you will notice that the 'new' session disappears when you navigate back to the first page.

Test script:
---------------
//test1.php
<form name="fileupload" action="test2.php" method="POST" enctype="multipart/form-data">
    <input type="hidden" name="<?php echo ini_get("session.upload_progress.name"); ?>" value="123">
    <input type="file" name="newfile">
    <input type="submit" name="submit" value="Submit">
</form>
<?php
    if(!isset($_SESSION)) {
        session_start();
    }
    print_r($_SESSION);
?>

//test2.php
<?php
    if(!isset($_SESSION)) {
        session_start();
    }
    $_SESSION['new']='Im a new value that will disappear';
    print_r($_SESSION);
?>
<a href="test.php">Go back</a>