Psycopg2 error at or near %
You have misplaced parenthesis.

with open(filename, 'rb') as f:
    f1 = f.read()
    cur.execute(
        "INSERT INTO testable (title, content, chron_date, clock) VALUES (%s, %s, %s, %s);", # remove parenthesis here, which ends the execute call
        (filename, f1, '2017-12-30', '23:59:00')
    ) # move above parenthesis here