Add support for (optional) file chunking

[rnicholus]

This is only possible with the XHR uploader. I should be able to make use of the slice function provided by the File API. This will greatly reduce the time needed to retry a failed upload, as we can simply retry starting with the last failed blob/chunk. The chunk size should probably be configurable.

Enabling this may also, theoretically, make it possible to overcome the upload file size restrictions (usually around 4GB) put in place by most browsers.