postgres=# CREATE FOREIGN TABLE foo () SERVER file;
ERROR:  filename is required for file_fdw foreign tables
postgres=# CREATE FOREIGN TABLE foo () SERVER file OPTIONS (format 'csv');
ERROR:  filename is required for file_fdw foreign tables
postgres=# CREATE FOREIGN TABLE foo () SERVER file OPTIONS (filename '', format 'csv');
CREATE FOREIGN TABLE
postgres=# ALTER FOREIGN TABLE foo OPTIONS (DROP filename);
ERROR:  filename is required for file_fdw foreign tables
postgres=# ALTER FOREIGN TABLE foo OPTIONS (DROP format);
ALTER FOREIGN TABLE
postgres=#