"/bla.bas" would be correct.
And the 250 line limitation is gone. 5000 lines can be used in a single program now. You may want to use the file manager to upload such a large program though and rename it to "/default.bas"
http://esp8266basic.com
A BASIC interpreter for your ESP
Working now on Autodrop3d. A 3d printer with automatic part ejection system. https://autodrop3d.com