If you want to hide a text box, you can put conditional formatting on it. For the first drop down, select The expression. In the text box after that drop down, clear out any content and type the number 1. Then select 'Hide this control'. This will always evaluate to true, thus the control will always hide.
One thing people sometimes struggle to understand about rules is that the field the rule is on needs to change for the rule to fire. So, if you want the file name field to default to today() if it is blank, you might want to consider setting that in an Open rule (under Tools / Form Options). Make sure you put a condition on it -- that file name should be blank!