Comment 1 for bug 1863018

a quick (and untested) way to fix the problem is modification of _build_uri function in mediafirebackend.py:

    def _build_uri(self, filename=None):
        u"""Build relative URI"""
        filename = filename.decode() if filename else None
        return (
            u'mf:' + self.folder[u"folderkey"] +
            (u'/' + filename if filename else u'')
        )