Convert base64 string to Blob

base64toBlob.js

function base64toBlob(b64Data, contentType, sliceSize) {
    contentType = contentType || '';
    sliceSize = sliceSize || 512;

    var byteCharacters = atob(b64Data);
    var byteArrays = [];

    for (var offset = 0; offset < byteCharacters.length; offset += sliceSize) {
        var slice = byteCharacters.slice(offset, offset + sliceSize);

        var byteNumbers = new Array(slice.length);
        for (var i = 0; i < slice.length; i++) {
            byteNumbers[i] = slice.charCodeAt(i);
        }

        var byteArray = new Uint8Array(byteNumbers);

        byteArrays.push(byteArray);
    }

    var blob = new Blob(byteArrays, { type: contentType });
    return blob;
};

//saving excel-file
$.post("api.yoursite.com", data, function (fileModel) {    
    var data = base64toBlob(fileModel.Base64Content, "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet;charset=utf-8");
    var filename = fileModel.FileName;
    saveAs(data, filename);
});