[解決済み] ファイル入力フォームからBase64文字列をjavascriptのファイルオブジェクトに変換する方法は？

 function dataURLtoFile(dataurl, filename) {
 
        var arr = dataurl.split(','),
            mime = arr[0].match(/:(.*?);/)[1],
            bstr = atob(arr[1]), 
            n = bstr.length, 
            u8arr = new Uint8Array(n);
            
        while(n--){
            u8arr[n] = bstr.charCodeAt(n);
        }
        
        return new File([u8arr], filename, {type:mime});
    }
    
    //Usage example:
    var file = dataURLtoFile('data:text/plain;base64,aGVsbG8gd29ybGQ=','hello.txt');
    console.log(file);

 //return a promise that resolves with a File instance
    function urltoFile(url, filename, mimeType){
        return (fetch(url)
            .then(function(res){return res.arrayBuffer();})
            .then(function(buf){return new File([buf], filename,{type:mimeType});})
        );
    }
    
    //Usage example:
    urltoFile('data:text/plain;base64,aGVsbG8gd29ybGQ=', 'hello.txt','text/plain')
    .then(function(file){ console.log(file);});