Android 3.0 – 4.2 之前的浏览器，包括微信浏览器等，都不支持 Blob 的构造方法，需要使用 BlobBuilder。

function newBlob (data, datatype) {  
  var out
  try {
    out = new Blob([data], { type: datatype })
  } catch (e) {
    window.BlobBuilder = window.BlobBuilder ||
    window.WebKitBlobBuilder ||
    window.MozBlobBuilder ||
    window.MSBlobBuilder

    if (e.name == 'TypeError' && window.BlobBuilder) {
      var bb = new BlobBuilder()
      bb.append(data)
      out = bb.getBlob(datatype)
    } else if (e.name == 'InvalidStateError') {
      out = new Blob([data], { type: datatype })
    } else {
      throw new Error('Your browser does not support Blob & BlobBuilder!')
    }
  }
  return out
}

// data URIs to Blob
function dataURL2Blob (dataURI) {  
  var byteStr
  var intArray
  var ab
  var i
  var mimetype
  var parts

  parts = dataURI.split(',')
  parts[1] = parts[1].replace(/\s/g, '')

  if (~parts[0].indexOf('base64')) {
    byteStr = atob(parts[1])
  } else {
    byteStr = decodeURIComponent(parts[1])
  }

  ab = new ArrayBuffer(byteStr.length)
  intArray = new Uint8Array(ab)

  for (i = 0; i < byteStr.length; i++) {
    intArray[i] = byteStr.charCodeAt(i)
  }

  mimetype = parts[0].split(':')[1].split(';')[0]

  return new newBlob(ab, mimetype)
}