Comment 3 for bug 1240215

So this seems to be the issue:

       # Any user specified filters such as arch~(amd64|i386) are in
        # addition to our selecting only tar.gz files. That's the only type
        # of file we know how to unpack.
        self.item_filters = item_filters or []
        self.item_filters.append('ftype=tar.gz')
        self.item_filters = filters.get_filters(self.item_filters)

MAAS has its arches as amd64/generic i386/generic (hence config file specifies ARCHES= "amd64/generic" and so on), while simple streams only handles filters such as "arch~(amd64|i386)", not considering the "/generic"