with sqlplus, how do i run an explain plan on a query in a file

generally, i can run a .sql file in windows like so:

SQL> @"longpath/sqlfile.sql"

and sqlplus will spit out the results of the query.

but i'm trying to run something like:

SQL> EXPLAIN PLAN FOR @"longpath/sqlfile.sql";

i'm getting an error

EXPLAIN PLAN FOR @"longpath/sqlfile.sql"
                 *
ERROR at line 1:
ORA-00905: missing keyword

not sure what keyword im missing...