User Matt has proposed the following code a year ago ( How to convert dataURL to file object in javascript? ) which might help you

EDIT: As some commenters reported, BlobBuilder has been deprecated some time ago. This is the updated code:

function dataURItoBlob(dataURI) {

  // convert base64 to raw binary data held in a string

  // doesn't handle URLEncoded DataURIs - see SO answer #6850276 for code that does this

  var byteString = atob(dataURI.split(',')[1]);



  // separate out the mime component

  var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0]



  // write the bytes of the string to an ArrayBuffer

  var ab = new ArrayBuffer(byteString.length);



  // create a view into the buffer

  var ia = new Uint8Array(ab);



  // set the bytes of the buffer to the correct values

  for (var i = 0; i < byteString.length; i++) {

      ia[i] = byteString.charCodeAt(i);

  }



  // write the ArrayBuffer to a blob, and you're done

  var blob = new Blob([ab], {type: mimeString});

  return blob;



}