Why is the Downloading file size unknown despite Content-Length headers set with file size in bytes?

Question

During Downloading files the size is unknown "0B" despite having set the "Content-Length" headers with the bytes (see $http_headers below)

Content-Type (String, 15 characters ) application/zip
Content-Disposition (String, 55 characters ) attachment; filename="test.zip"
Content-Length (Integer) 454732390
Pragma (String, 8 characters ) no-cache

FYI - code for file transfer (using Taxonomy File Tree)

$file = file_load($node->tft_file[LANGUAGE_NONE][0]['fid']);
    $http_headers = array(
      'Content-Type' => $file->filemime,
      'Content-Disposition' => 'attachment; filename="' . $file->filename . '"',
      'Content-Length' => filesize($file->uri),
    );
    if (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE')) {
      $http_headers['Cache-Control'] = 'must-revalidate, post-check=0, pre-check=0';
      $http_headers['Pragma'] = 'public';
    }
    else {
      $http_headers['Pragma'] = 'no-cache';
    }

file_transfer($file->uri, $http_headers);

Is the format wrong or am I missing something?