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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def files(self, ftype=None):
 """ return a first of path to all files within that folder """

        a = [self.join(i) for i in self]
        if ftype is not None:
            return [i for i in a if i.isfile() and i.filetype() == ftype]
        return [i for i in a if i.isfile()]