Update
BlobBuilder has been obsoleted see how you go using it, if you're using it for testing purposes.
Otherwise apply the below with migration strategies of going to Blob, such as the answers to this question.
As an alternative there is a Blob that you can use in place of File as it is what File interface derives from as per W3C spec:
interface File : Blob {
readonly attribute DOMString name;
readonly attribute Date lastModifiedDate;
};
The File interface is based on Blob, inheriting blob functionality and expanding it to support files on the user's system.
Using the BlobBuilder like this on an existing JavaScript method that takes a File to upload via XMLHttpRequest and supplying a Blob to it works fine like this:
var BlobBuilder = window.MozBlobBuilder || window.WebKitBlobBuilder;
var bb = new BlobBuilder();
var xhr = new XMLHttpRequest();
xhr.open('GET', 'http://jsfiddle.net/img/logo.png', true);
xhr.responseType = 'arraybuffer';
bb.append(this.response); // Note: not xhr.responseText
//at this point you have the equivalent of: new File()
var blob = bb.getBlob('image/png');
/* more setup code */
xhr.send(blob);
The rest of the sample is up on jsFiddle in a more complete fashion but will not successfully upload as I can't expose the upload logic in a long term fashion.