拿到blob和转换

gistfile1.txt

function getObjectURL(file) {
        var url = null;
        if (window.createObjectURL != undefined) {
          url = window.createObjectURL(file)
        } else if (window.URL != undefined) {
          url = window.URL.createObjectURL(file)
        } else if (window.webkitURL != undefined) {
          url = window.webkitURL.createObjectURL(file)
        }
        return url
      }
      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 });
      }