def add_file(self, fileGrp, mimetype=None, url=None, ID=None, pageId=None, force=False, local_filename=None, **kwargs):
        """
        Add a `OcrdFile </../../ocrd_models/ocrd_models.ocrd_file.html>`_.

        Arguments:
            fileGrp (string): Add file to ``mets:fileGrp`` with this ``USE`` attribute
            mimetype (string):
            url (string):
            ID (string):
            pageId (string):
            force (boolean): Whether to add the file even if a ``mets:file`` with the same ``ID`` already exists.
            local_filename (string):
            mimetype (string):
        """
        if not ID:
            raise Exception("Must set ID of the mets:file")
        el_fileGrp = self._tree.getroot().find(".//mets:fileGrp[@USE='%s']" % (fileGrp), NS)
        if el_fileGrp is None:
            el_fileGrp = self.add_file_group(fileGrp)
        if ID is not None and self.find_files(ID=ID) != []:
            if not force:
                raise Exception("File with ID='%s' already exists" % ID)
            mets_file = self.find_files(ID=ID)[0]
        else:
            mets_file = OcrdFile(ET.SubElement(el_fileGrp, TAG_METS_FILE), mets=self)
        mets_file.url = url
        mets_file.mimetype = mimetype
        mets_file.ID = ID
        mets_file.pageId = pageId
        mets_file.local_filename = local_filename

        self._file_by_id[ID] = mets_file

        return mets_file