def __init__(self, files={}, data={}, callback=None, deferred=None):
self._files = files<EOL>self._file_lengths = {}<EOL>self._data = data<EOL>self._callback = callback<EOL>self._deferred = deferred<EOL>self.boundary = self._boundary()<EOL>self.length = self._length()<EOL>
Initialize.

        Kwargs:
            files (dict): A dictionary, where key is the field name, and value is the file path
            data (dict): Additional data to post
            callback (func): Callback to inform progress (receives sent, and total)
            deferred: Deferred to call when done, or when error occurs