I’d be 99% sure that it’s the result of a hack. When I’ve seen file names in the past that that “magically” appear like that, they normally are.
You’ll most likely see base64_decode() somewhere in that file, probably close to the top. That will confirm that it’s a malicious file.
http://codex.wordpress.org/FAQ_My_site_was_hacked
I just upgraded to WP 3.6 and it seems to have gone
However, in the file I saved, base64_decode does turn up About halfway down, where the following code appears’
if(!function_exists('full_urlencode')) {function full_urlencode($p){$r='';for($i=0;$i<strlen($p);++$i)$r.= '%'.dechex(ord($p[$i]));return strtoupper($r);}}
$stringTools = array(
'Base64 encode' => 'base64_encode',
'Base64 decode' => 'base64_decode',
'Url encode' => 'urlencode',
'Url decode' => 'urldecode',
'Full urlencode' => 'full_urlencode',
'md5 hash' => 'md5',
'sha1 hash' => 'sha1',
'crypt' => 'crypt',
'CRC32' => 'crc32',
'ASCII to HEX' => 'ascii2hex',
'HEX to ASCII' => 'hex2ascii',
'HEX to DEC' => 'hexdec',
'HEX to BIN' => 'hex2bin',
'DEC to HEX' => 'dechex',
'DEC to BIN' => 'decbin',
'BIN to HEX' => 'binhex',
'BIN to DEC' => 'bindec',
'String to lower case' => 'strtolower',
'String to upper case' => 'strtoupper',
'Htmlspecialchars' => 'htmlspecialchars',
'String length' => 'strlen',
);
It’s a big file with a lot of scary words (like “Brute”) hopefully i got it before it did any harm.
