file對象轉換爲Blob對象

function dataURLtoBlob(dataurl) {
    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 Blob([u8arr], {
        type: mime
    })
}