blobToBase64(blob) {
        const reader = new FileReader;
        const p = new Promise( (success, fail) => {
            reader.onloadend = () => {
                let base64 = reader.result;
                success(base64);
            };
            reader.onerror = (e) => {
                fail(e);
            }
        });

        reader.readAsDataURL(blob);

        return p;
    }
Convert Blob object to base64string
@param {Blob} blob
@returns {Promise} success(base64 string), fail(error)