PHP file download returns page not found

using cURL in PHP to call the /files/download api.  Dropbox returns "Error (4xx) We can't find the page you're looking for."  How to troubleshoot? I think I am using the API correctly.

function getTextFile( $token )
{
  $ch = curl_init();

  curl_setopt($ch, CURLOPT_URL, "https://content.dropboxapi.com/2/files/download");
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
  curl_setopt($ch, CURLOPT_POST, 1);
  curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0 ) ;

  $headers = array();
  $headers[] = "Authorization: Bearer " . $token ;
  $headers[] = "Dropbox-Api-Arg: {\"path\": \"/test_upload.txt\"}";
  $headers[] = 'Content-Type: text/plain; charset=utf-8' ;
  curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

  $result = curl_exec($ch);
  if (curl_errno($ch)) {
      echo 'Error:' . curl_error($ch);
  }
  curl_close ($ch);
  return $result ;
}