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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 attach(self, attachment, filename=None, ctype=None):
 """ attach a file :param attachment: File to attach, given as :class:`~alot.db.attachment.Attachment` object or path to a file. :type attachment: :class:`~alot.db.attachment.Attachment` or str :param filename: filename to use in content-disposition. Will be ignored if `path` matches multiple files :param ctype: force content-type to be used for this attachment :type ctype: str """


        if isinstance(attachment, Attachment):
            self.attachments.append(attachment)
        elif isinstance(attachment, str):
            path = os.path.expanduser(attachment)
            part = helper.mimewrap(path, filename, ctype)
            self.attachments.append(Attachment(part))
        else:
            raise TypeError('attach accepts an Attachment or str')

        if self.sent_time:
            self.modified_since_sent = True