Change __%s__ to {{%s}} in rename

Hello,

Currently there is an issue with how files are renamed.
Often I need to be able to rename files whose names begins with `_`,  right now it fails.

What I propose is to change rename sequence to `{{%s}}`, because:
* it's more rare that we would want to have file begins with `{`
* it's more consistent, we already use `{{}}` in templates as variables