When you use a "at scheduled time" trigger, there is no triggering record. Thus, you cannot include record data. Instead you need to tell your automation what record to use. You could include a "Find Records" action to find the record(s) with file you want to attach. Then you will be able to the file from the record.
Note that in the body of the email, you can include only a link to the file. And that link will either require someone to be logged into Airtable with access to the record, or the link will expire in a few hours. To actually include the file as an attachment, you will have to specify the attachment field in the attachment section of the email action.
