Unable to use form fields in attachment file name

I am trying to use a dynamic filename as an attachment to mail(2). It will not work. Your documentation says this is OK:

[your-file][your-another-file]

I am creating a CSV file containing the fields in the form and have done that successfully using form fields to create the name. I want to pass this file in the email. When I try using form fields in the attachment field, it simply ignores it. I know my path is correct because a static named file works fine, it just will not use the form fields I specified as the name of the file. I am passing a random number (and yes, it is getting through correctly) as part of the file name: ABC-[randomnumber].csv is what I am trying to use.