Filters to be used within the convert filter are base64-encode, base64-decode, quoted-printable-encode and quoted-printable-decode. Note: those are not in the string filter, as currently reported by the manual!
Example usage is:
<?php
$h = fopen('gecodeerd.txt', 'r');
stream_filter_append($h, 'convert.base64-decode');
fpassthru($h);
fclose($h);
?>
Or
<?php
$filter = 'convert.base64-decode';
$file = 'coded.txt';
$h = fopen('php://filter/read=' . $filter . '/resource=' . $file,'r');
fpassthru($h);
fclose($h);
?>
stream_get_contents