FormData and BlobBuilder - duplication of functionality?

Hi,

It seems we are currently implementing 2 very similar objects -
FormData<http://dev.w3.org/2006/webapi/XMLHttpRequest-2/#the-formdata-interface>
 and BlobBuilder <http://www.w3.org/TR/file-writer-api/#idl-def-BlobBuilder>.
Both can be created, then strings or Blobs can be appended, then it can be
sent via XHR or saved into a file (well, FormData can not be saved in the
file yet). But, they are very similar:

var bb = new BlobBuilder();
bb.appendText("Lorem ipsum");
bb.appendBlob(someBlob);
xhr.send(bb.getBlob());

var fd = new FormData();
fd.appendText("data1", "Lorem ipsum");
fd.appendBlob("data2", someBlob);
xhr.send(fd);

Are those two similar enough to justify merging them into a single object
(defined as a a list of named BlobItems) that could be used for both?

One difference is that BlobBuilder does not use a mime wrapper around data
pieces, but rather concatenates them - there could be a property for that.

Dmitry