upload php form help

so i am trying to use a form to upload pdf documents to my server remotely. The php code is in its own file called "upload.php". The error I am getting is the die error "The file you attempted to upload is not allowed" therefore something is wrong because I have permissions set for PDF documents which is what I am trying to upload.

<?php
   // Configuration - Your Options
      $allowed_filetypes = array('.pdf'); // These will be the types of file that will pass the validation.
      $max_filesize = 524288; // Maximum filesize in BYTES (currently 0.5MB).
      $upload_path = './users/'; // The place the files will be uploaded to (currently a 'files' directory).
 
   $filename = $_FILES['userfile']['name']; // Get the name of the file (including file extension).
   $ext = substr($filename, strpos($filename,'.'), strlen($filename)-1); // Get the extension from the filename.
 
   // Check if the filetype is allowed, if not DIE and inform the user.
   if(!in_array($ext,$allowed_filetypes))
      die('The file you attempted to upload is not allowed.');
 
   // Now check the filesize, if it is too large then DIE and inform the user.
   if(filesize($_FILES['userfile']['tmp_name']) > $max_filesize)
      die('The file you attempted to upload is too large.');
 
   // Check if we can upload to the specified path, if not DIE and inform the user.
   if(!is_writable($upload_path))
      die('You cannot upload to the specified directory, please CHMOD it to 777.');
 
   // Upload the file to your specified path.
   if(move_uploaded_file($_FILES['userfile']['tmp_name'],$upload_path . $filename))
         echo 'Your file upload was successful, view the file <a href="' . $upload_path . $filename . '" title="Your File">here</a>'; // It worked.
      else
         echo 'There was an error during the file upload.  Please try again.'; // It failed :(.
 
?>

here is the HTML form code

<center><table width="600" align="center" cellpadding="5">
<form action="upload.php" method="post">
<p><font color="red"><?php print("$message");?></font></p>


<tr>
      <td width="163"><div align="right">File:</div></td>
      <td width="409"><input type="file" name="userfile" id="file"/></td>
    </tr>
            <tr>
      <td><div align="right"></div></td>
      <td><input type="submit" name="Submit" value="submit" /></td>
    </tr> </form>
    
</table>
</center>