You can specify FILE_NAME_CONVERT = NONE, which is the same as omitting this clause. If you omit this clause, then the database first attempts to use Oracle Managed Files to generate file names. If you are not using Oracle Managed Files, then the database uses the PDB_FILE_NAME_CONVERT initialization parameter to generate file names. If this parameter is not set, then an error occurs.
Reference:
https://docs.oracle.com/database/121/SQLRF/statements_6010.htm#SQLRF55686