replace_last_occurrence.js

function str_replace_last(search, replace, str){
    let pos = str.lastIndexOf('webp');
    let search_length;
    if(pos != -1){
        search_length = search.length;
        str = str.substring(0, pos)+replace+str.substring(pos+search_length);
    }
    return str;
}

str_replace_last("webp", "jpg", "22708867webp360e747bwebp444651.webp");