无法通过自定义PHP上传器上传CSV文件

我试图制作一个非常简单的php上传器，只上传一个.CSV文件，我需要它来覆盖服务器上已经存在的文件。我试着处理上传部分，直到它工作起来。我已经在上传目录中设置了777权限，但在上传文件时不断出错。覆盖部分我还不能编码它，我可能需要一些帮助来制作它。.csv文件是1.5MB。上传文件时我得到的错误是说文件的扩展名错误，但我110%确信扩展名是.csv。

这是代码：

上传.php

  <?php
  // Configuration - Your Options
  $allowed_filetypes = array('.csv',); // These will be the types of file that will pass the validation.
  $max_filesize = 5000000; // Maximum filesize in BYTES (currently 5MB).
  $upload_path = 'upload/'; // Upload 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 a *.CSV file.');
   // Now check the filesize, if it is too large then DIE and inform the user.
   if(filesize($_FILES['userfile']['tmp_name']) > $max_filesize)
  die('The file you attempted to upload is larger than 5MB.');
   // Check if we can upload to the specified path, if not DIE and inform the user.
   if(!is_writable($upload_path))
  die('You cannot upload to the specified directory, please CHMOD it to 777.');
   ?> 

HTML:

<form enctype="multipart/form-data" action="upload.php" method="GET">
Update CSV File: <input name="uploaded_file" type="file" />
<input type="submit" value="Upload" />
</form>