Use MD_STD::unlink over unlink in mime type check

src/MD_STD.php

@@ -594,7 +594,7 @@ public static function remote_mime_content_type(string $url):string {
         \curl_close($ch);
 
         $mime_type = MD_STD::mime_content_type($tmp_file);
-        \unlink($tmp_file);
+        self::unlink($tmp_file);
 
         return $mime_type;